Nothing is stopping you from installing the stable version, is there? However if you saved projects in a pre-release version there is no guarantee they will work in the previous releases. Or future ones for that matter.
Isn't this already a known issue? https://github.com/LMMS/lmms/issues/851
It seems to be an issue with Xorg or Qt rather than LMMS. I wonder if LMMS could work around it...I have no experience with GUI programming so I can't say more about that.