This has been happening since my game updated to 1.16.

So when you switch to another app or lock the screen and go back to the game, the Minecraft loading screen goes up, then the pause screen.

However in some cases, the screen glitches out and doesn't go back to normal. It would seem as the screen were split in half, and I think the controls were affected too, as the buttons on moving rarely work.

The screenshot attached is an example