In KeepassXC, which I use in an offline capacity as it appears you do, has CTRL+B as it's cut and paste login, and CTRL+C as it's cut and paste password. Both with a tiny window of staying alive for about 5 seconds as you toggle from their app to the browser login fields to paste..
if you can do this in Chrome instead of FF, then it's a FF problem and you've at least ruled something out.
if it fails in both, then you have a repo package problem, but is it a timeout problem as mentioned above?
if flatpak worked flawlessly, why did you switch to repo version? or are you saying it worked in Flatpak until recently? like yesterday? if that's the case that's the 4th flatpak horror story I've read here in 24 hours...if this doesn't match your situation please disregard.
see anything else about this on the internet? (bitwarden plus no cut and paste)
you have to start eliminating variables here to sharpen the diagnosis/prognosis, as it were.
...of course the first person to replicate your problem will nullify all my hypotheses🙂.