Piglins do not serialize the number of ticks until they are converted to Zombified Piglins, meaning that repeated serialization and deserialization of the chunk they are in will cause their conversion time to continuously reset, causing them to never zombify. This behavior is different from that of the drowned, which serializes the DrownedConversionTime field.

Looking at the decompiled code, the piglins store a field names timeInOverworld which increments each time they are in the overworld, yet does not get serialized or deserialized into the NBT.