I posted this in a vanish bug thread already.
There is a video of a rogue showing that vanish doesn’t work correctly.
https://us.forums.blizzard.com/en/wow/t/video-of-the-vanish-bug/
In the end you can see him dropping combat inside of BRD and just restealthing (without vanish) while mobs are still following him.
Seems like there is a huge bug with combat inside dungeons atm.