Boss bug Derelict Mansion

I was following wisps on the atlas going from map to map to unlock the wisp map and got derelict mansion i walk into the boss room to try and fight it and avelyne is dancing while vaarloch plays the piano and i cant trigger the boss fight ive logged out and died and tried everything i could to get the fight to start and nothing is working my main problem is that i cant retry the map after its over please fix

got the same issue, and I think I know what causes the bug. If Fate of the Vaal spawns a boss, the game thinks the actual map boss fight has started. the unique fight music started playing too, as soon as the boss became damageable. so when you enter the arena, the game thinks the fight is already over.
a workaround for this would be to rush to the boss before interacting with any mechanic, not just Fate of the Vaal, since I remember I had a similar issue with Rituals being too close to the boss room last league where starting a ritual could teleport one or both bosses out of the arena, and when you start the boss fight later, the boss wouldnt return to the arena and thus the fight couldnt be completed.