Colors in Manipulate[] plot change during execution

UPDATE WITH WORKING EXAMPLE

Following what @Kuba said, I’ve attempted to find a MWE that shows the problem without too much of the other stuff, and here’s what I’ve done.

Here’s a couple of new GIFs to show the problem: one of them is a "closeup" to what happens at the 50th point threshold, let’s say; the other one is the entire computation.

Now for the code used!

I’ve moved the red coloring up front, so it’s clearer.
Bear with the initial numerical lists, they’re needed to compute the plot!

Needs["ComputationalGeometry`"]

ax = {-0.1746015783681`, -0.171744280521`, -0.1753424479885`, 
-0.1818458390149`, -0.1809263260599`, -0.179514865692`, 
-0.1855975041929`, -0.1946914824968`, -0.1950397333495`, 
-0.1927830485728`, -0.1903676037906`, -0.1813196070505`, 
-0.1791299762296`, -0.1829230554782`, -0.1773529674489`, 
-0.1787983200338`, -0.1879380539756`, -0.182404296226`, 
-0.1784390293998`, -0.1860209123609`, -0.1809745997407`, 
-0.1704765474764`, -0.1715602192745`, -0.1760999618028`, 
-0.1800162507619`, -0.1839354357525`, -0.1674646656581`, 
-0.1600203030248`, -0.1707130354576`, -0.1776024218527`, 
-0.1822423762589`, -0.1909958033058`, -0.187934546206`, 
-0.1751694881397`, -0.1672043083525`, -0.1756886117798`, 
-0.1786703799603`, -0.1643010527693`, -0.1609161369298`, 
-0.1548654897932`, -0.159330579992`, -0.1666642778476`, 
-0.1705566606098`, -0.1651841230109`, -0.1590624597891`, 
-0.1572528899478`, -0.1645688614408`, -0.1691789062584`, 
-0.1700776661369`, -0.1831444320553`, -0.1756400139901`, 
-0.1726808740354`, -0.1839756338981`, -0.1850826110625`};

ay = {0.0331378591656`, 0.0409481302281`, 0.0422659823338`, 
   0.043104907643`, 0.0470038392703`, 0.0493021015212`, 
   0.0572258911383`, 0.0617176993451`, 0.0717445961243`, 
   0.074826655056`, 0.0701977897732`, 0.0744846137707`, 
   0.0782133209711`, 0.0765184550871`, 0.0704380146454`, 
   0.0675561618231`, 0.0654454440966`, 0.0646604678239`, 
   0.0610291942788`, 0.050425044872`, 0.0481746091749`, 
   0.0459573319292`, 0.0333965852916`, 0.0235605891572`, 
   0.0117588371722`, 0.0033762281309`, 
   0.0006015510757`, -0.0161951821118`, -0.0239303359718`, 
-0.0243545255201`, -0.0302308027512`, -0.0326944750102`, 
-0.0372133126888`, -0.0392181142279`, -0.0446498649591`, 
-0.048496237962`, -0.0521995787495`, -0.0597592798179`, 
-0.0639631319254`, -0.0612661252642`, -0.0555731192592`, 
-0.053499608459`, -0.047099952508`, -0.034003132337`, 
-0.0301277803374`, -0.0234019856993`, -0.0159851047869`, 
-0.0094231411377`, -0.010157386318`, -0.0095515292143`, 
-0.0058310425886`, -0.0077447972646`, -0.0095447993443`, 
-0.0110744762862`};

Dimensions[ax]

Dimensions[ay]

g = 9.81;
ch = ConvexHull[Transpose[{ay, ax}]];
chf = Transpose[{ay, ax}][[ch]];
chf = Join[chf, {First[chf]}]; (* to close the plot *)

Manipulate[
           car  = 1 ;; pos;
            Show[
                ListLinePlot[chf],
                
                ListPlot[
                    Transpose[{ay, ax}],
                    PlotStyle -> {ColorData["HTML"]["DarkGray"]},
                    PerformanceGoal -> "Speed"
                        ],
  ListPlot[
                        Transpose[{ay[[car]], ax[[car]]}],
                        PlotStyle -> {ColorData["HTML"]["Red"]},
                        PerformanceGoal -> "Speed"
                        ],
            ImageSize -> 750,
            AspectRatio -> 1,
            PlotRange -> Automatic,
            Frame -> True,
            FrameLabel -> {{"[LeftArrow] Longitudinal acceleration ->",
                            "<- Longitudinal acceleration ->"}, 
                           {"<-Left Curve/ Right Curve ->", 
                            "<- Left Curve/ Right Curve ->"}}
            ],
 {{pos, 1, "Position"}, 1, Length[ax], 1}, Paneled -> False
 ]

What I’ve found from this is that the problem doesn’t arise if the plot contains less than 50 points (so in the example 54 are used), and the change in color appears exactly when the 50th point is colored in red.
The notebook has its own context, I’ve deleted all unnecessary stuff. While writing this, I thought that maybe Performance -> "Speed" could have been part of the problem, but deleting it doesn’t change anything, it seems.

If it’s easier to see firsthand, here’s the MWE notebook. No other files needed.

ORIGINAL QUESTION

Don’t know if the title is clear, but I’ve got a video to explain better.
[EDITED OUT because it was wrong and it wasn’t showing the problem]

As you can see, the colors I’m using in the left plot seem to change of intensity/depth/other during the animation execution.
I thought this could depend on the "heaviness" of the computation, but even PerformanceGoal -> "Speed" seems not to have any effect on that.

Here’s the part of code I’m using to generate that plot. I’ll make the whole notebook available if it’s needed, but since it’s a good amount of computations to get to this point, for now I’ll just leave this snippet.

Manipulate[
 car  = 1 ;; pos;
 quot = Quotient[pos, 705];
 index = quot + 1 ;; quot + 2;
 curve = curves[[quot + 1]] ;; curves[[quot + 2]];
 circuit = Transpose[{xCircuit, yCircuit}];

 Row[{[
     Show[
          ListLinePlot[chf],

          ListPlot[
                   Transpose[{ayfil[[car]]/g, axfil[[car]]/g}],
                   PlotStyle -> {ColorData["HTML"]["Red"]},
                   PerformanceGoal -> "Speed",
                   PlotLegends -> 
                       Placed[SwatchLegend[{"evolution"}, LegendMarkerSize -> 15, 
                       LegendMarkers -> "Bubble", 
                       LegendLabel -> Style["Curves", 20]], Left]
          ],

          ListPlot[
                   Transpose[{ayfil/g, axfil/g}],
                   PlotStyle -> {ColorData["HTML"]["DarkGray"]},
                   PerformanceGoal -> "Speed",
                   PlotLegends -> 
                       Placed[SwatchLegend[{"g-g plot"}, LegendMarkerSize -> 15, 
                       LegendMarkers -> "Bubble"], Left]
          ],
    
          ImageSize -> 750,
          AspectRatio -> 1,
          PlotRange -> {{-3.5, 3.5}, {-4, 2}},
          Frame -> True,
          FrameLabel -> {{"<- Longitudinal acceleration ->",
                          "<- Longitudinal acceleration ->"}, 
                         {"<- Left Curve/ Right Curve ->", 
                          "<- Left Curve/ Right Curve ->"}},
          LabelStyle -> Directive[Black, Bold]
     ]
     ,
     Show[
          ListLinePlot[
                       Transpose[{xCircuit, yCircuit}],
                       ImageSize -> 500, 
                       AspectRatio -> Automatic
          ],

          Graphics[{
                    PointSize[0.02],
                    Green,
                    Point[circuit[[pos]] ],
                    PlotLegends -> Placed[{"real-time position"}, Left]
          }]
     ]
 }],
{{pos, 1, "Position"}, 1, Length[axfil], 50, AnimationRate -> 200}, 
Paneled -> False]

Can’t figure out what to change or add unfortunately!

EDIT: forgot to mention I’ve got Mathematica 11.3 on Windows 10
EDIT2: since I’m not sure how to post a simpler MWE without pasting a bunch of numbers, here’s the whole notebook and xls files needed, if this helps… I thought the code itself could be the problem, but I guess having a proper working program at hand is always better -> notebook

(* setup code from question *)

Manipulate[
           car  = 1 ;; pos;
            Show[
                ListLinePlot[chf],
                
                ListPlot[
                    Transpose[{ay, ax}],
                    PlotStyle -> {ColorData["HTML"]["DarkGray"]},
                    PerformanceGoal -> "Speed"
                        ],
                ListPlot[
                        Transpose[{ay[[car]], ax[[car]]}],
                        PlotStyle -> Directive[ColorData["HTML"]["Red"], AbsolutePointSize[7]],
                        PerformanceGoal -> "Speed"
                        ],
            ImageSize -> 750,
            AspectRatio -> 1,
            PlotRange -> Automatic,
            Frame -> True,
            FrameLabel -> {{"[LeftArrow] Longitudinal acceleration ->",
                            "<- Longitudinal acceleration ->"}, 
                           {"<-Left Curve/ Right Curve ->", 
                            "<- Left Curve/ Right Curve ->"}}
            ],
 {{pos, 1, "Position"}, 1, Length[ax], 1}, Paneled -> False
 ]

GeneralUtilities`PrintDefinitions@Charting`adaptivePointSize

size = Which[
    n<50,
        7,
    n<100,
        6,
    n<250
        5,
    n<500
        4,
    n>15000,
        1,
    ...
]