Relates to MC-172255. Most likely intentional. Piglins don't dismount on the entity they're riding, even if they have the NoAI set to 1b.
Endermen with NoAI still teleport when hit
Slimes/magma cube with NoAI-Tag still can damage the player
Bees with NoAI can get angry
Piglins immediately dismount any entity that they are riding