The bug

1.21 snapshots have finally allowed for entity-riding-entity stacks to use portals. However, this only works if the bottom-most entity in the stack touches the portal - upper members of stacks will not be affected at all.

How to reproduce

1. Clip a minecart into an obsidian block using MC-8004
2. Use this obsidian as a bottom block of a nether portal
3. Light the portal then enter the minecart

Expected results

Since you are intersecting the portal, you would be taken through it. You would either go through it alone leaving the minecart behind, or (preferably) the whole stack would go through.

Actual results

You are not teleported at all.