Tikzscale does not scale a figure when it contains addplot3

I wanted to implement a 3d-figure containing stacked 2d-layers, with an example below:

I used the following code, with a main file:

documentclass{article}
usepackage{standalone}
usepackage{tikz}
usetikzlibrary{external}
usepackage{pgfplotstable}
usepackage{pgfplots}
usepackage{tikzscale}
pgfplotsset{compat=1.17}
begin{filecontents*}{file1.dat}
    x y
    0 0
    1 1
    2 2
    3 3
    4 4
    5 5
end{filecontents*}
begin{filecontents*}{file2.dat}
    x y
    0 0
    1 4
    2 6
    3 8
    4 10
    5 12
end{filecontents*}
usepackage{subcaption}
tikzexternalize[prefix=tikz-cache/]
tikzset{external/force remake}
usetikzlibrary{pgfplots.groupplots}

pgfplotsset{every axis/.append style={
        label style={font=footnotesizebfseries},
        tick label style={font=footnotesize},
        legend style={font=footnotesize}
    },
    area plot/.style={
        fill opacity=1,
        draw=black!50!black,
        fill=white,
        mark=none,
    },
    y axis/.append style={align=center}}
tikzset{Line Label/.style={font=footnotesize,scale=2}}
newcommand{figurefontsize}{footnotesize}

begin{document}
    begin{figure}[htpb]
        centering
        includegraphics[width=.5linewidth]{sub.tikz}
        caption{Image I}
    end{figure}
end{document}

and the tikz-file sub.tikz:

pgfplotstableread{file1.dat}{tablea}
pgfplotstableread{file2.dat}{tableb}
tikzsetnextfilename{sub}
begin{tikzpicture} 
    begin{axis}[
        ymin=0, ymax=30,
        xmin=0, xmax=5,
        xlabel={$x$},
        ylabel={$y$},
        grid=major,
        legend entries={(y_1),(y_2),(y_1+y_2)},
        legend pos = north west
        ]
        % Select appropriate columns
        
        addplot3 [area plot, ultra thick, red] table [x=x, y expr=2, z=y] {tableb}closedcycle;
        addplot3 [area plot, ultra thick, blue] table [x=x, y expr=1, z=y] {tablea} closedcycle;
    end{axis}
end{tikzpicture}

Now, when compiling, the figure does not change size, regardless if I use

includegraphics[width=5linewidth]{sub.tikz}

or

includegraphics[width=.5linewidth]{sub.tikz}

The same approach works if I have 2d-data. Why does it fail for the current approach?