I am unsure of the exact nature of this bug, but I am willing to assist with a world download if necessary. The description is as the title states.
I brought 3 cats, all on leads, into a Nether portal so I could transport them to my house. I had previously done this with a single cat with no issue. However, when all 3 of them enter the Nether, and then I attempt to enter the Nether, the Realm crashes. When it restarts, the cats have completely disappeared, nowhere to be found. Bringing them in 1 at a time will not crash the Nether, even though all 3 are present.
This is reproducible the other way around as well; that is, if all 3 cats EXIT the Nether and then I also exit the Nether, the Realm will crash with the same result. This may be related to odd behavior exhibited by cats who enter the Nether while attached to a lead (they will glide when sitting as if attached to a lead, and cannot be made to sit without reattaching the lead).
I have a world download if necessary to help reproduce and troubleshoot the bug, although it is too large to attach. I reproduced this bug three separate times while on the Realm. Notably, this bug does NOT seem to be reproducible in a single player world; the game does NOT crash and I can bring all three cats into the Nether just fine.
Please let me know if I can provide any additional information to assist in troubleshooting this bug.