I faced the same issue. It has something to do with video adapter drivers. When my desktop is displayed by an intel video card - I also have problems with displaying elements, but if you have an nvidia video card in your computer, then either set its mode to “performance” in the drivers, after which the entire desktop will be drawn by a discrete video card, or “On-Demand” mode, but in this case, in the shortcut to the shotcut program, you need to add nvidia-optimus-offload-glx, you get something like this line: “nvidia-optimus-offload-glx /usr/bin/flatpak run --branch=stable --arch=x86_64 --command=shotcut --file-forwarding org.shotcut.Shotcut @@ %F @@”, after that - shotcut will work fine.
You can wait a little longer and a version based on qt6 will be released soon, this problem is not there, I tried to run the night version, and there are no display problems. But hardly anyone can say when it will be released exactly, but judging by the activity on the github in the qt6 branch, work on the new version is in full swing and seething, I follow this branch.