This just happened to me and hour or so ago. I was going to fight two Withers, one after another. The first Wither behaved appropriately I guess, nothing weird. Second Wither dashed straight down at half health, carving a perfectly square hole about 60+ blocks (was at about y75, Wither dashed down to y15).
It went straight down into a huge cavern, and it was a pain to finish the fight, with all the blocks laying around from the first Wither, and then from this one going straight down, the game became very laggy.
My son is having the same problem on his
Xbox One SXbox One X. When Launching Minecraft, the game takes over 5 minutes just to load to the main menu, and another 5-ish just to load a world.I do not have this issue on my phone, my PC, or his iPad.
This is on the most recent version, 1.20