The bug
If there isn't a nether portal in the nether, but mobs do try to go to the nether, they will get deleted.
To reproduce
Create a new world
Create a nether portal, but don't go through it
Push a villager into the portal
Observe the villager staying in the overworld
Go through the portal and back
Push a villager into the portal
Observe it go through
Linked issues
Comments 2
Thank you for your report!
This seems to describe the same issue as MC-151648. Although this ticket has been reported earlier, the latter has more info and is being updated more frequently. For that reason, we're forward resolving this as a duplicate.
If you'd like to add any additional info over there, that would be appreciated!
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Issue no longer present in 1.13.1. Tested with villager, wolf (wild), wolf (pet), and sheep and all mobs spawned in the nether before I did.