I believe the included image speaks for itself. The default Typst font is the one on the left, and Fira Math is the one on the right. Regarding the default font, where the overscript lines have extended as expected, there also appears to be a mismatch with the arrow heads, but it turns out that's just what happens with the default font weight used in Typst. So that's something to watch out for if you decide to implement such a feature. This issue about extending (vector) arrows most like isn't limited to Typst, otherwise I wouldn't bother sharing it.

For comparison, here's what it looks like typeset in LaTeX:

\usepackage{harpoon, unicode-math}

\begin{document}

$\overrightharp{ABCDEFGHIJK}$

\setmathfont{Fira Math}

$\overrightharp{ABCDEFGHIJK}$

\end{document}

I sort of went overboard with the length of the contents, but it's obvious that both fonts have the arrows extended. As far as I'm aware, it's the harpoon package which is responsible for extending the arrows. It also skews them, but again, you're probably not going to have arrows that long. Actually, it probably only skews them instead of separately adding the line segments as done in the Typst example.