Sometimes when entering or exiting the Nether or the End some of the chunks will load invisible, and mobs will be frozen. Player can still move, but cannot access any menu/inventory and the loading bar animation will play in centre of screen continuously. Only solution so far is to force reset Minecraft and start again. Once when this happened i waited 15+minutes and the day/night cycle continued and player character could move but not interact with anything. This also happens sometimes when flying with elytra and firework rockets, so possibly something to do with chunk generation??? only started happening with 1.16.0.64