Boss fight not starting when bosses are part of abyss

Map: Derelict mansion

Objective: Defeate Varloch and Avelyne.

Issue: Abyss pit in boss room. Cleared whole map. Boss fight not starting, they are just playing piano and dancing. Can't clear abyssal pit because bosses are the last monsters for it.

same issue! so frustrating after doing several maps to unlock the encounter :(