There seems lately a big issue with alot of mobs that are invisible. Even up to an invisible wither (which of course is hard to fight). Forum-search showed, that a similar problem was in skyfactory 2 and that it was fixed. Maybe it's possible to fix it for regrowth, too?
In most cases this works for me, too. (though it can be sometimes difficult first finding a hiding spot, since logging out and in while under attack is not a good idea). In case of the wither relogging didn't help. Had to leave the nether and go back in to see it again.
