아래 코드의 \fill 부분에 \ftone, \fttwo 로 하면 에러가 나는 반면에 \def에 해당하는 문구로 바꾸면 정상적으로 출력됩니다. 원인이 무엇인지 알고 싶네요.

\documentclass[12pt]{standalone}
\usepackage{kotex}
\usepackage{mathtools}
\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{calc,math,angles,positioning,intersections,decorations.markings,backgrounds}
\tikzmath{%
\xmax=3;
\xmin=-3;
\ymin=-5;
\ymax=2;
\pp=\xmin+0.2;
\qq=\xmax-0.2;
\aa=-2;
\bb=0;
\cc=2;
}%

\begin{document}
\begin{tikzpicture}[yscale=0.4, xscale=1.2,
declare function={
F1(\x)=pow(\x,4)-4*pow(\x,2);
F2(\x)=0;}
]
\draw[line width=0.6pt, -stealth, name path global=lx] (\xmin,0)--(\xmax,0) node {$x$};
\draw[line width=0.6pt, -stealth, name path global=ly] (0,\ymin)--(0,\ymax) node {$y$};

\def\ftone{plot[domain={\aa:\cc},samples=300] (\x,{F1(\x)})} %\ftone 의 정의
\def\fttwo{plot[domain={\cc:\aa}] (\x,{F2(\x)})} %\fttwo 의 정의

\begin{scope}
\clip (\xmin,\ymin) rectangle (\xmax,\ymax);
\draw[very thick,name path global=f1] plot[domain={\pp:\qq},samples=300] (\x,{F1(\x)});
\draw[thick,name path global=f2] plot[domain={\pp:\qq}] (\x,{F2(\x)});
\end{scope}

\begin{scope}[on background layer]
\fill[gray!60] (-2,0)--\ftone--\fttwo--cycle; %에러가 나는 부분
%\fill[gray!60] (-2,0)--plot[domain={\aa:\cc},samples=300] (\x,{F1(\x)})--plot[domain={\cc:\aa}] (\x,{F2(\x)})--cycle; %정상적인 출력
\end{scope}
\coordinate (O) at (0,0);
\coordinate (Ox) at (1,0);
\coordinate (Oy) at (0,1);

\end{tikzpicture}
\end{document}