I tried to draw an area under a function, but it results a strange shape.
\documentclass{article}
\usepackage{tkz-fct}
\begin{document}
\begin{tikzpicture}[scale=.7071]
\tkzInit[xmin=0,xmax=5,ymin=0,ymax=5]
\tkzGrid[color=white]
\tkzDrawX[label=$\Delta t$,noticks]
\tkzDrawY[label=$\Delta s$,noticks]
\tkzFct[domain=0:5,samples=100,id=vt]{2}
\tkzDrawArea[domain=1:4]
\end{tikzpicture}
\end{document}
I thought of a rectangle shape. Tried the solution given in question problem with surfaces on tkz-fct , but the problem still exists. My tkz-fct version is 1.4c by the .sty header

gnuploton PATH, and compiled with-shell-escape. Also tested on overleaf's texlive 2021, and the output is ok too. – muzimuzhi Z Nov 30 '21 at 02:23\listfilesto preamble, run, and paste the version info from end of.logto the question. So I can see if versions of any included files are different. – muzimuzhi Z Nov 30 '21 at 10:28tkz-fct? Also can you run your example on overleaf, download the output PDF, then open and check it shown as expected? – muzimuzhi Z Dec 03 '21 at 09:59