Sacred Resevoir Boss Bug

Was running a sacred res, got to boss fight and started to dps, It cut to the animation where whoever touches the boss, suddenly he disappears? and I am stuck in the boss room unable to tp out or obviously get the loot

I have now had this bug twice, once during campaign and once in maps.

Issue appears to be if you are never inside the circle of purple swords he can charge off screen and despawn. cannot kill the boss, boss room is still active, only way of getting out is relogging.

Still shows as 1 monster remains, except the boss has despawned or possibly teleported into the wall (I cannot hear any sounds effects from him anywhere in the room, so probably despawned)

re-entering the map does not resolve the issue.