Wither skeletons and zombie pigmen won't go through lava to get to enemies, even though they can't burn

This is most likely because Wither Skeletons share the same type of AI as a regular Skeleton as far as I know, and the regular Skeleton can burn when touching lava... so apply that to the Wither Skeleton, even if immune, and they won't dare to touch the lava. This is my guess anyway!

That is correct IIRC. The only way this could be altered is by splitting the entities (making them seperate)

On another note, this might be WAI. Do WSes take damage from the lava itself? Recall that burning is only a side effect, and the lava itself also can cause damage.

Chris: Wither Skeletons are immune to fire, lava and daylight.

This is what I'm talking about! Wither skeletons should be unique mobs and not mob subtypes. Wither skeletons need spawn eggs and their own unique behaviors.

Zombie pigmen also do this in the latest snapshots, now that they pathfind and see you 40 blocks away. It's not that they share the same AI as skeletons and zombies, it's that they were updated to the ganeral "smart" mob AI. As it is, every mob with it doesn't pass through lava, and these two mobs in particular have been left unchanged, which makes no sense obviously. Skeletons and wither skeletons are understandable, but for the zombie pigmen, being a unique mob (except the parts shared by zombies), they have potential to have a unique AI. I hope this kind of AI gets implemented into them. On second thought, maybe they can only do it when there is no way to go around the lava, since lava is pretty slow anyway.

Can confirm for 1.8 pre1

Confirmed for 1.9-pre4

Still in 1.9.1-pre1

Still in 1.9.1-pre3

Still in 1.9.3-pre2

Confirmed for 16w42a

Please do not mark unreleased versions as affected.
You don't have access to them yet.

Still in 18w20a

Still in 18w22c

Still in 18w50a