I spawned a Wither in the 1.14 release, and was surprised to find it attacking every mob I spawned, including undead ones, which it's not supposed to do.
I tested with wither skeletons, zombies, phantoms and zombie pigmen. I also tested ghasts due to the wiki saying that it doesn't attack them (despite them not being undead), and strangely, they are the one mob it didn't attack.
Can confirm.