Zombified Piglins spawn using the old model (zombie pigman) on all my worlds that I made before 1.16. The betas released prior to 1.16 did not have this issue. I have only been able to recreate this on 1.16.20.50. Zombified Piglins spawned using both the spawn egg, or the portal relighting method have the same model of Zombie Pigmen (see attached). Loading in new chunks also spawns them with the old model. This also affects their baby counterparts.

I'm not sure if this affects all world that were created prior to 1.16 or just the worlds on my device. I have so far been unable to verify this due to the lack of people on the betas who I can speak with.

Picture 1: I used spawn eggs

Picture 2: I rapidly broke and relit portals to spawn Zombified Piglins