Initial troubleshooting steps:
1. Reset LR's preferences: https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/ . LR often soils its preferences, and resetting them can fix a surprising number of bad behaviors.
2. If that doesn't help, please copy the first ten lines of the menu command Help > System Info. You may be the victim of a nasty upgrade bug.