Sometimes the fourth boss stops taking any damage. It does not become inactive, there is no relevant buff, no "immune" or "absorb" messages. Just the hp remains the same no matter which attacks we use. Happened several times by now.
This may be caused by the boss being silenced before going into the "Phanotm Anubis" phase but we cannot tell for sure yet.
PS. Seems that instance bugs are too frequent, you may want to consider making a dedicates section or category for them.