I tested with Firefox Snap, Vivladi (apt) and Microsoft Edge (Flatpak).
The moment my scroll wheels scrolls a bit, the file choose suddenly closes.
I’m on Plasma 6 Wayland. I have yet to test on X11.
I have never encountered this on Unstable Edition, and I have used Firefox (snap) and accessed the file chooser frequently.
Not sure if I’m missing something.
https://bugs.kde.org/show_bug.cgi?id=481972
Edit: I’ve confirmed that, by going back to X11, it doesn’t happen anymore. I’ll stay here in the meantime (as this is a work computer).
Strange that I’ve never encountered this on my Unstable edition.