This issue is already tracked as a KDE bug, but mentioning it here in case anyone finds this and it can shorten their bug-hunting journey.
In Fedora KDE 42, Filelight won’t show pie charts when using nVidia drivers. (The app is functional, it still calculates and shows the directory usage textually, it just doesn’t render the pie chart.)
This issue is only seen with Qt 6.9, so you won’t find it in earlier Fedora releases. For example, Fedora 41 has Qt 6.8, so you won’t see the bug there.
KDE bug tracker link: 502709 – Pie chart on the right is invisible with Qt 6.9, NVIDIA graphics, and Filelight running as a Wayland application
nVidia forum link (issue raised by KDE developers): KDE App Filelight broken in Wayland + Qt 6.9 or newer - Linux - NVIDIA Developer Forums