The bug

You can spawn piglins riding on other entities, but when you try to spawn a piglin riding on a hoglin or a piglin, they dismount. They can stay riding on zoglins however.

This is probably due to the playing behavior of a baby piglin and baby hoglin where the baby piglin decides to stop riding on the hoglin. With commands, the piglin rider will always decide to dismount when it rides on a hoglin/ other piglins on top of each other.

How to reproduce

1. Run

   /summon ravager ~ ~ ~ {Passengers:[{id:piglin,HandItems:[{id:crossbow,Count:1b}]}]}
   

   → The piglin stays sitting on the ravager

2. Run

   /summon hoglin ~ ~ ~ {CannotBeHunted:1b,Passengers:[{id:piglin,HandItems:[{id:crossbow,Count:1b}]}]}
   

   → Its piglin rider automatically dismounts the summoned hoglin.