In TeX StackExchange (https://tex.stackexchange.com/questions/412804/why-can-lualatex-not-overlay-a-white-number-over-a-tikz-node) report that XeLaTeX does print a white number over a grey background positioned with TikZ, using the instruction 'color=white' instead of 'text=white'. The other engines (pdfLaTeX and LuaLaTeX), overley a white rectangle over the grey background with 'color=white' but not with 'text=white'.
Acording to user TeXnician, " It is documented that a color option overrides the text color and the draw color, so the colors would become white in the OP. And limiting the scope of the color change to the text seems to remove this problem (as I would expect following the documentation)."
Therefore, without technical knowledge of TeX, XeTeX and TikZ, I suspect this is a bug or a carasteristic of XeLaTeX, comparing the outputs of the three engines: pdfLaTeX, XeLaTeX and LuaLaTeX.
Anonymous
I think that the problem is in the TikZ driver
pgfsys-dvipdfmx.def used by XeTeX.