Updated description by GoldenHelmet

Steps to reproduce

1. Build a nether portal in the overworld that crosses a chunk border.
2. Go through the nether portal.
3. Make sure that at least the south or east half of the portal in the nether corresponds to overworld coordinates that are south or east of the portal in the overworld.
4. Go back through the southern or eastern half of the portal in the nether.

Expected result

The nether portal in the overworld stays intact.

Actual result

The nether portal in the overworld partially breaks.

Note: as this is due to loading lag, you can increase the chance of this happening by increasing the loading burden. What triggered it for me was:

• Run Minecraft in full-screen mode.
• Set render distance to max.
• Drop lots of items in the chunk with the south or east portion of the portal.

Increasing simulation distance and random tick speed, and using the portal with a 2nd LAN player, did not trigger a portal break.

Original description

So, in my Realms world, I spent a decent amount of time putting together a nice little cave-looking entrance to my 11x8 nether portal with a diorama behind it (blocked by glass blocks) so it looks like you're looking into the nether as you're entering the portal... It looked exactly how I wanted it to, and worked as expected for a day or so.

Yesterday, (~18 hours after using it successfully several times) I entered the nether, and, upon coming back to the overworld, my portal was...wonky. Take a look at the images... As you can see, it's no longer filling the entire frame. I've broken and re-lit it several times over the course of the past 24 hours. Each time I relight it, it's full-sized, but every time I use it to go into the nether and then back into the overworld, it looks like this...

The only added detail I have, relates to when/how it happened... I entered the nether and there were two zombie pigman standing on the portal blocks there. I immediately went back into the overworld to see if I could find them there, and I found that I was holding one of their swords and my portal was broken as shown in the link... It's like my portal somehow glitched, killed the pigman, and the sword dropped into my hand...? No idea. Could it be an chunk-border issue?