The bug

If a piglin is on a minecart and when it converts into a zombified piglin, it sinks into blocks. It also happens with the baby piglins that convert into zombified piglins which makes them suffocate into the blocks.
This bug also works if the minecart is boosted by the powered rails, which results the converted zombified piglin to sink into blocks. (See the video below that says "The Result but with the powered rail method")

How to reproduce

1. Put rails and a minecart
2. Spawn a piglin
3. Put the piglin on the minecart (make sure the minecart doesn't move and be placed on full-block spot, or the bug will not work. Or have the minecart boosted by the powered rails)
4. Wait for it to convert into a zombified piglin
   → Notice it sinks into blocks