I just checked again and I'm still having the problem mentioned in https://bugs.mojang.com/browse/MC-83284 so I actually get the exact message that is shown in that issue's screenshot.
When looking in the client and server logs there is no notice of anything that went wrong, all I can see is that I disconnected, so I can't really give you more info than that...
I had the same problem in a multiplayer server; after respawning the ender dragon (which didn't spawn upon first entering the end), no boss bar showed up for neither me or my friend.
I just checked and I can't even reproduce it myself!
Either I imagined it or there's something specific that needs to happen for it to go wrong, either way I'd suggest ignoring or closing the issue for now.
IF it comes up again, I'll be sure to take a screenshot next time 😉