I have a few zombie pigman in a one block deep trench filled with lava. Whether in survival or creative, the pigman will not move around no matter what. I have tried hitting them and they still won't move I waited for around five minutes. They attempt to move because they have the animation of moving up and down when swimming.

Steps to Reproduce:
1. Build a one block deep lava trench.
2. Spawn zombie pigment down in the lava.
3. Notice that they will not move, no matter how long you wait. Try going into survival and hitting one of them.