Affects me on elementary OS 5.0 "Juno", which uses Gala as its window manager. I also have an AMD R9 380, so I'm betting this is solely a LWJGL 3.2.1 problem and not caused by KDE.
Anybody have a workaround? I tried MultiMC, but it seems to have a problem switching LWJGL versions for 1.14.
This bug has resurfaced for me in 1.14.4 as well. I'm using Pantheon, which, again, is what elementaryOS 5.0 "Juno" uses as a desktop environment. This bug was not present while playing 1.14.3 through the official launcher, so there must have been a regression at some point during development of 1.14.4.
A workaround for me is to play it fullscreen, but Pantheon gives a fullscreen window a new set of workspaces, which makes multi-tasking to access wikis or my music player really obnoxious.