When you are fighting a raid or the ender dragon with your friends it is really fun. Except when you can't see the bossbar. If you are in a boss fight and you go through any portal then the bossbar will not appear making it almost impossible to see your progress. My friends and I encountered this bug in our realm. I went through a nether portal during a raid to my underground village and the raid bar did not appear, but upon going out of the village and back it did appear for me. The same happened in the end. We were fighting the dragon and I died and went back and the bar just disappeared. This happened to all of us eventually and soon no one new how much health it was at. This was very stressful. We had 4 people online at the time three on split screen and one on another switch.

Recreate

1. Summon a raid while another player is in the nether.
2. Make that player come back through a portal into the village perimeter.

Results the second player will not have a bossbar. You may have to do this multiple times to get it to work. This can work with any portal not just nether. This cannot be fixed in the end so it basically makes the dragon fight really annoying. It can happen with any bossbar including wither.