Mirage Chain spawned in an inaccessible boss room

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Haunted Mansion map, the mirage area barely touched the door to the basement(boss room). However, the area transition didn't spawn, preventing me from going to the boss room. On the mini-map the mirage area extends to the south, which according to the map from the original area is where the boss room is located.

The mirage chain is located nowhere inside the accessible area, so it must be inside the boss room that I can't go inside.