When a mob enters the Overworld by a Nether portal, it will return to the Nether

At first, I planned to move villagers to an underground location, so I used a Nether portal to transport them. After successfully settling the villagers, I played for a while and later noticed that they were gone. Initially, I thought they had despawned, but I later discovered that they had all automatically returned to the Nether.

Below is my video demonstration using a cow as an example. When I send the cow to the Nether and then leave the chunk it is in, exit the game, and rejoin, I find that it has been transported back to the Nether.

This seems to occur probabilistically and does not happen every time.

Reproduction Procedure:

1. Transport villigers by a Nether portal.

2. After confirming that the villagers, the Iron Golem, and the livestock in the village are working properly, leave the village.

3. After leaving the chunk where the village are located, then exiting the game and rejoining.

4. Then visit the village again.

Expected outcome:

The village has proper mob-proofing in place, so no hostile mobs can spawn that would harm the villagers. Additionally, the villagers have no way to leave the area they are in, so it is impossible for them to enter the Nether portal on their own.

As a result:

The village is empty.

And all of them were automatically transported back to the Nether portal through which they were originally brought.