Wrong font scaling using when groupplot, tikzscale and fillbetween, maybe a bug (?)

Situation:

I am trying to place one bigger pgfplot at the left side of a figure and four small plots (2×2) on the right half, like this:

|--------|----|----|  
|        | P2 | P3 |  
| plot 1 |----|----|  
|        | P4 | P5 |  
|--------|----|----|  

For this I use two groupplots next to each other as shown here: Create groupplot 3 x 6. I scale them with tikzscale, in order to fill the page and still maintain fontsize etc. Furthermore I need to plot one legend above all plots, as explained here: PGFplots – label each group plots

Problem:

Everything works fine as long as I do not use fillbetween in the pgf plot. Here is a MWE that I compile twice with LuaLaTeX under Windows (MiKTeX, all packages are updated), the fillbetween commands are commented so far:

documentclass{standalone}
usepackage{pgfplots}
usepgfplotslibrary{groupplots}
usepgfplotslibrary{fillbetween}
usetikzlibrary{calc}
usetikzlibrary{matrix}
usepackage{tikzscale}
pgfplotsset{compat=newest}
usepackage{filecontents}
begin{document}
begin{filecontents}{A.tikz}
begin{tikzpicture}
  begin{groupplot}[
  group style={
    group name=left plot,
    group size=1 by 1,
  },
  width=0.45*textwidth,
  height=6cm,
  xlabel={$x$},
  ylabel={$y$},
  scale only axis,
  xmin=-0.028,
  xmax=0.2222,
  ymin=0.0,
  ymax=0.6666,
  xmajorgrids,
  ymajorgrids,
  axis x line=bottom,
  axis y line=left,
  ]

  nextgroupplot[]
  addplot[name path=OneA,solid,color=gray, forget plot] coordinates { (-0.027,0.18) (0.0,0.0) (0.222,0.6666)};

  addplot [name path=TwoA,solid,color=gray, forget plot] coordinates { (-0.027,0.18) (0.222,0.6666)};

  %addplot[lightgray, forget plot] fill between[of = OneA and TwoA];

  addplot coordinates { (0,0.05) (0.05,0.2) (0.15,0.55) (0.2,0.4) }; label{plot1}
  addplot coordinates { (0,0.6) (0.05,0.1) (0.15,0.26) (0.2,0.44) }; label{plot2}
  addplot coordinates { (0,0.1) (0.05,0.3) (0.15,0.33) (0.2,0.2) }; label{plot3}
  addplot coordinates { (0,0.42) (0.05,0.41) (0.15,0.4) (0.2,0.39) }; label{plot4}
  addplot coordinates { (0,0.6) (0.05,0.02) (0.15,0.61) (0.2,0.32) }; label{plot5}

  end{groupplot}

  begin{groupplot}[
  group style={
    group name=right plots,
    group size=2 by 2,
    horizontal sep = 1cm,
  },
  width=0.2*textwidth,
  height=2.1cm,
  scale only axis,
  xmajorgrids,
  ymajorgrids,
  ]
  nextgroupplot[anchor=north west, at={($(left plot c1r1.north east) + (2cm,0)$)},
  xmin=-0.015,
  xmax=0.08,
  ymin=0.0,
  ymax=0.22,]
  addplot[name path=OneB,solid,color=gray, forget plot] coordinates { (-0.027,0.18) (0.0,0.0) (0.222,0.6666)};

  addplot [name path=TwoB,solid,color=gray, forget plot] coordinates { (-0.027,0.18) (0.222,0.6666)};

  %addplot[lightgray, forget plot] fill between[of = OneB and TwoB];

  addplot coordinates { (0,0.05) (0.05,0.2) (0.15,0.55) (0.2,0.4) };
  nextgroupplot[]
  addplot coordinates { (0,0.05) (0.05,0.2) (0.15,0.55) (0.2,0.4) };
  nextgroupplot[]
  addplot coordinates{ (0,0.05) (0.05,0.2) (0.15,0.55) (0.2,0.4) };
  nextgroupplot[]
  addplot coordinates { (0,0.05) (0.05,0.2) (0.15,0.55) (0.2,0.4) };
  end{groupplot}

  % legend
  path (left plot c1r1.north west|-current bounding box.north)--coordinate(legendpos)(right plots c2r2.south east|-current bounding box.north);
  matrix[
  matrix of nodes,
  anchor=south,
  draw,
  inner sep=0.2em,
  draw
  ]at([yshift=1ex]legendpos)
  {
    ref{plot1}& $a=1$&[5pt]
    ref{plot2}& $a=2$&[5pt]
    ref{plot3}& Lit
    ref{plot4}& $a=3$&[5pt]
    ref{plot5}& $a=4$
  };

  end{tikzpicture}%
end{filecontents}

includegraphics[width=0.9textwidth]{A.tikz}
end{document}

It produces the (expected) output:

When I uncomment the first fillbetween command in plot 1, font scaling crashes:

And things become really scary when I activate fillbetween in the first of the four smaller plots (P2). This leads to wrong font scaling and wrong placement of the plot:

So I am not sure whether this is some kind of bug or whether I am just doing something wrong. Every help is greatly appreciated.