"Improper alphabetic constant" error when input standalone subfile

Upgrade to TexLive2020 recently, and get "Improper alphabetic constant" error when input a subfile with the standalone pkg. There’s no problem if the subfile contents are directly put in the main file. There seems also no problem if the subfile is regular without begin{document}...end{document}.

The file compiled fine with old TexLive2019. It seems if I change the class to article, it goes fine. But I write in Chinese, the ctexart class is much more convenient. Please help.

Here’s a MWE.
Main file:

% !TeX encoding = UTF-8
documentclass{ctexart}

usepackage{standalone}

usepackage{tikz}

begin{document}
    input{G105}

    begin{tikzpicture}
        coordinate (O) at (0,0);
        coordinate (A) at (1,1);
        fill[gray] (O) |- (A) -- cycle;
    end{tikzpicture}
end{document}

Subfile: G105.tex

% !TeX encoding = UTF-8
documentclass[class=ctexart]{standalone}
usepackage{tikz}

begin{document}
    begin{tikzpicture}
    coordinate (O) at (0,0);
    coordinate (A) at (1,1);
    fill[gray] (O) |- (A) -- cycle;
    end{tikzpicture}
end{document}

documentclass[fontset=fandol]{ctexart}

usepackage{standalone}

usepackage{tikz}

begin{document}
%ShowHook{env/document/before}
RemoveFromHook{env/document/before}[xeCJK] 
    input{G105}

    begin{tikzpicture}
        coordinate (O) at (0,0);
        coordinate (A) at (1,1);
        fill[gray] (O) |- (A) -- cycle;
    end{tikzpicture}
end{document}