Aren't there KWin rules to capture the mouse? Maybe that could be a workaround.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
IDK, current plan is to do a reinstall and see if that helps.
I've had that issue with Deep Rock Galactic anytime I opened the Steam Overlay. No idea if it's related, but if you're using Steam you could disabling that.
I'm already past nuking my system and I'm setting everything up from scratch. I hope the issue won't come back again, because I didn't have it pre upgrade to Plasma 6 and I use Overlay a lot.
Report bugs to https//bugs.kde.org, not here. Thread locked.