Simply put: We got through a fortress to get to the End Portal, and found a mostly complete nether portal in the lower floors. As a means of getting back to the base quicker we completed the portal and found our way to the existing nether portal. We have some time to toss everything into a chest/start smelting/etc. After what I would guess is 20 mins to an hour the server will disconnect. Setting progress back to an hour (sometimes more than that), I assume it is the save point prior (and maybe two back if a certain number of reverts have happened). It seems to crash faster if we attempt to go back through the portal to the nether (moving items to main base from fortress).

This is on a realms hosted server, I have not seen this bug happen on any other version of Minecraft (might be luck, idrk). Could be a realms issue is what I am stating, but I imagine it is more related to a bug in the Caves and Cliffs update.

I assume this is an issue with the save of the world. The only question is why would this logically occur if there are more than 1 nether portals.

I have tried to move the nether portal by like 5 or so blocks. I could try to move it further but where the portal spawns is relatively close to the other portal (at main base).

At times this bug causes some form of lag, not sure if it is ping/etc, not sure how to diagnose that. Several hours have been spent trying to see if there is a way to not cause this crash.

If there is something I can do to give you more info, feel free to suggest/request info.