With the new mob AI update, Pigmen will no longer pathfind through lava to get to the player, even though pigmen are immune to lava.

How to replicate:
1. Create a new super flat world in creative and turn mob spawning off (/gamerule doMobSpawning false)
2. Build a lava moat around yourself.
3. Spawn a pigman on the other side
4. go into survival and attack it
5. Pigmen will act slow when trying to attack you, and will also not go into the lava.