It's less of a bug than an issue but I still think it's an oversite. Whenever I fight the wither and it gets to its second phase where it breaks a lot of blocks easily, it makes a thousands of block entities from all the broken block items. While this is normal, if the fight goes on for too long, there will be thousand of entities present lagging  the game to the point where it's very hard to fight the boss. This (like I said) isn't a glitch but it's a very anoying issue. Maybe have the blocks drop into item form less frequently when destroyed by the wither. This would be very helpful and make the fight smoother.