Abyss monsters not spawning, boss fight won't start, Derelict mansion

Bug report reference: 3,693,684,287

There was an abyss in the boss room, last few monsters from the abyss never appeared and so the boss is bugged and the boss fight won't start. /remaning shows 4 monsters, searched the entire map (upstairs and down) multiple times, can't seem to trigger anything. Avelyne just keeps dancing around with her skeleton friend.