After receiving update 1.13.1, I went to the nether and teleported to my blaze spawner. Upon arrival, I was greeted by what seemed like (almost) every possible mob - in high numbers - in the nether. To list a few that I found... mooshroom, cow, pig, Llama, sheep, horse(with diamond armour, presumably my horse but never taken it to the nether), enderman, Vindicator, squid(no water), zombie, creeper, etc, etc...
In addition, I also found an armour stand from my home base, my trained dogs inside minecarts, some cats, and other various items.
I have also found that every one of our item frames, decorations, and picture frames have vanished or yet to be found. However, our signs are still in place.
Note for screenshots:
Nether images were taken after killing enough mobs to prevent us from lagging out, which was a lot.
*_21.15.29.png is an image of our empty barn, which seems to be where all of the non-aggressive mobs in the portal came from.
*_21.15.49.png & *_21.15.55.png are images of where the armour stand came from and empty walls of what used to be filled with item frames.
If necessary, I can provide the information to the realm in which this incident has occurred.
Looks like MC-136815 was confirmed for vanilla.