Summary

Villagers sometimes disappear upon being sent from Nether to Overworld.

Testing

I have done a small amount of testing on other worlds with both naturally spawned, and player spawned villagers. The test was conducted several times on each world. Newly created worlds, and worlds that were created in earlier game versions, were tested. Multiple nether portals were tested in each world.

Conclusion

It appears that out of the 4 worlds the only world experiencing this issue is my main one (created in a earlier game version), the villagers in other worlds seemed to travel to and from the nether just fine.