The problem:

There seems to be an issue when it comes to boats and minecarts, lets say a shulker for example, you retrieved it from an end city and you are bringing it back into the overworld, and then the boat/minecart wont go through the portal.

This applies to the nether portal, making mob transportations impossible to move one place to another, if a minecart or boat has no entities ridden inside, they go through the portal just fine, but doesn't allow mobs to go through ridden.

A screenshot below shows proof of creepers as examples of them in minecarts and boats thus not going through portals, not to mention it also applies to players as well.

This can lead to a lot of frustration to players if they are attempting to do the "How did we get here" advancement.

How to reproduce:

1. Load of a superflat world and create two nether and end portals separately

2. Use any mob that can be ridden on minecarts/boats

3. Put the mob on a ridden minecart/boat into the portal and see results