Very specific issue, but I'm using immersive portals 1.20.1 fabric on forge via sinytra connector. (yes i know it's not supported) I have a dimension stack world, and once you cross a portal, the plane (I tested the battleship) goes through and keeps you, but all mobs onboard fall out. (and probably die due to fall damage).
Don't know if this issue persists on native forge iportals.

And would a boat work?

ah, yes they still fall out in boats. seems to be an Iportals issue. mb

Maybe they were lazy and only consider the primary passenger :)