Updated steps from GoldenHelmet: this is not related to collision or preloading the other dimension. It seem to be triggered by the entity being pulled into a ride the moment it loads.

Steps to reproduce

1. Link portals in the overworld and enther.
2. Next to one of the portals at the north or west side, place a minecart or boat.
3. In the other dimension spawn a mob next to the portal and push it in. If using a boat in step (2), make sure it is a mob that can go in boats.
4. Go through the portal.
5. Move the minecart/boat away from the portal.
6. Break the minecart/boat.

Expected result

The rider exits the minecart/boat where you break it.

Observed result

The rider teleports back to the portal.

There are different types of methods to be able to make an attraction to contain the entity and it cannot teleport, not only with a minecart or boat but also by mounting this, I have tried it with llamas, if you want to try an extra method the entity must be in a minecart, boat or you are mounting the entity as a flame, you have to move it away from the portal until you download the chunks of the portal so that the entity can no longer detect the portal and when you dismount or break the minecart/boat it will not teleport but at the moment you The entity detects the portal again, that is, you load the chunks and approach the entity and it will teleport if you are not using a boat, minecart or you are riding the entity. In summary, there are 3 types of containment so that the entity cannot teleport if it is close from the portal, and one that is moving the entity far enough away from the portal that I didn't detect it. How this report uses a containment method that is the minecart/boat to reproduce the problem, then the mounting is done, it is simple before doing step (5) mount the entity and then break the minecart/boat but without dismounting to the entity since when you are mounting and at the same time the entity is in a minecart/boat you are using 2 content methods if you break the minecart/boat then you would be having only 1 containment method which is mounting, in short if you have already broken the minecart/boat and you want to dismantle the entity, but you don't want it to teleport then use containment method 4 which is to move the entity away from the portal enough so that it doesn't detect it, if you used a flame you will have a lot of difficulty getting the entity away. call or break the minecart/boat, what you can do is have a friend help you keep the entity away with a leash and have him break the minecart/boat.

Reproduce MCPE-174930 in overworld.mp4 video

Steps to reproduce in overworld:

1. Open MCPE-174930.mcworld
2. Break minecart in nether
3. enter nether portal
4. Do not allow the entity to touch the portal for at least 15 to 20 seconds before it can enter or it will remain in the overworld and will not cross the portal. It will only teleport. (Additional step)
5. Break minecart in overworld

Observed results:
The entity teleports back to the portal

Expected results:
The entity not back to the portal