So, this is the second time this happened to me.
First time it happened, the server was about to reset, so we all assumed it was because of the server reset.
However, I’ve seen some reddit posts of this happening to someone else, but he was in a closed realm, so they assumed it happened because the realm closed right when the boss died.
This time, the realm was on ents, and nothing was about to be reset, and the realm was not closing or about to be closed.
All three instances, including the person who posted this on reddit, was a forever rubberband disconnect.
I’m suspecting there is a new bug with the boss dying that results in a disconnect.
Update since i posted this, there has been a flood of posts, pics/vids included, on reddit, of people disconnecting after boss kill, so hopefully, this has become a big enough issue to get the attention for it to be patched in the next update.