I was playing snapshot 21w05a and testing features when suddenly I realized that some actions in the game stopped working even though the game wasn't lagging. All the block updates, mob spawning, mob moving, randomticks, gui opening, commands, eating etc. are broken. Only I can do is move, break or place blocks and open inventory. Day cycle and light updates work. First time this happened to me I closed the game and everything I did while being in this lag mode, didn't save.