I had the same issue suddenly noticed today. I had to mess around/reboot a lot. I didn’t even have Meta + … input registering when trying to assign shortcuts. Then after removing all shortcuts (yeah, one guy said he had four, I had two application launcher configurable shortcut + the one in the actual application launcher for whatever reason), changing keyboard to another one and back to generic 105 keys, I had Meta key registering again so I add Alt+F1 shortcut to the application launcher settings, then in System Settings->Shortcuts->Application launcher I change the first one to Alt+F1 too and it started working again…
Another KDE special treat lol