The bug

When riding an entity through a portal, the playersNetherPortalDefaultDelay and playersNetherPortalCreativeDelay game rules are disregarded, making it inconsistent with teleporting through portals alone.

How to fix

It may make sense to change nether portal delay from a game rule to an entity attribute, which would be 80 for players and 1 for other entities by default. Moving a stack through a portal could use the highest value found within the stack.