When playing in full screen in Minecraft: Java Edition, for some reason, a cursor will appear in the middle of the screen, in which it is really annoying, it can be triggered when leaving any type of UI, but more common when leaving the chat screen.
Reproduce:
Load game in fullscreen
Open chat box or GUI
Close it, and mouse should appear.
Please check if that's still an issue for you in the latest 1.14.4 stable release or the latest 1.15 development snapshot.