When opening the launcher the game loads in fine, but Mojang loading screen appears on both monitors (only supposed to be on left one). Then, game screen will appear on both monitors, but game only is playable on left monitor, but only controlled from the screen on the right (which shows a frozen minecraft game). However this is only the case in fullscreen mode. In windowed mode, the game functions normally and only appears on one screen. With only one monitor plugged into the computer, everything works as normal in windowed or fullscreen mode. This issue is new to me with the 1.14.4 update.
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There was also a Reddit thread about it 2 months ago, which resurfaced a few days ago again as the issue seems to be reoccurring: https://www.reddit.com/r/Minecraft/comments/bj123c/dual_monitor_glitch/.
OK I figured it out. This issue seems to occur when the monitor marked as primary in Windows/Nvidia settings is not the monitor with the number 1. Since you can't change the numbers manually, the only way to do that is to literally switch which ports the monitors are plugged into on the GPU.
My primary monitor (left) was assigned number 2 originally, and my secondary (right) was assigned number 1. It looks like Minecraft always tries to go to monitor number 1 when going fullscreen, even if it is not the primary monitor. And especially in this case it causes the weird mouse-running-on-other-screen behaviour.
I am also getting this issue, just the other way with the screens. Left is my main monitor, right is secondary. Game launches, and plays on the right one for some reason, and is frozen on left, but only responds to cursor movements on the left monitor.
It's really difficult to describe, so I made a video: https://www.youtube.com/watch?v=xfshKZk4pxQ.