When the game is minimalized and it just loaded in the fullscreen setting, the window is cursed. When I put it on the front, it was a small borderless window. I reapplied fullscreen. But then my minecraft was forced to the front meaning that I couldn't multitask anymore.
I discovered this on 1.16.4 but it also happens on 1.16.5. A possible fix is that the game does not apply fullscreen when it is minimalized.