Overworld mobs can't go out of the Nether.

Does this only happen on a Realm or does it also occur in a local copy of the world?
Have the mobs moved out of the portal area before trying to return to the Overworld?

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Does this only happen on a Realm or does it also occur in a local copy of the world?
Yes, this occurs on a local copy (directly downloaded from the Realm backups) and on our Realm server.

Have the mobs moved out of the portal area before trying to return to the Overworld?
Yes, I think they have. The mobs have entered the Nether in a portal located around 2,5 fully zoomed out in-game maps away. We ran to the portal inside the Nether which takes us back home, skipping the aforementioned distance. However, crossing that portal was not possible for the pets. Not our home portal and not the portal they entered the Nether with. This should be out of the portal area considering that both portals aren't linked but I don't know.

If needed, I can try to create a video. I'd like to know what exactly I'd need to include before I start to really go into this process if possible.

Okay, after another bit of testing I found the following:

The portal that was far away was weirdly linked with another one. So, there are 2 portals at the further away location. These portals are fairly far apart. Half a map size of a fully zoomed out map.
We activated an already spawned portal which brought us back to the first portal in the Nether. After breaking the that spawned portal on the overworld, the pets started teleporting with me again immediately after entering the portal again with no issues.

All of this happened in singleplayer on the local copy and in creative mode.

Are you saying the bug no longer occurs?

I'm unsure. What I'm sure about is that the mobs definitely did not want to go through the portal and in our Realm we still have that issue. I can try destroying one of the portals again and see what happens, since that seemed to have done something in singleplayer.

Maybe this is due to the link these 2 portals have on the overworld? Talking about the 2 that are further away from our home. The 2 portals that are further away are linked in a way that we can always enter the Nether through both portals, spawning at the same Nether portal but only come out on the first overworld portal of those 2. Destroying the first one resulted in the pets teleporting to the portal in the Nether. So I assume that is the fix? 

Affects 1.20.15. I think I may have some information about this bug. I had this issue with an Elder Guardian, and using an NBT editor, I found that it's "PortalCooldown" was set to 299. So the issue seems to be that mobs don't have their portal cooldown go down correctly.

Based on the comment about two overworld portals linking to the same nether portal, I am resolving this report as a duplicate of MCPE-166172.