Oh, I just encountered a bug. I am trying to bring Nessiah and Rhyme together, but when I get to the Infernal Caverns, the game will freeze if I touch a lava tile and it also won't let me go back up the stars. However, using the crystal or a returner to exit (therefore exiting the quest) will allow the lava tiles to work again. I tried it going through the quest up to that point twice on the same save file and it definitely is tied to escorting Nessiah. It seems like Nessiah is treated like a party member to some extent. I tried using Malevolent Flame on the first area and I still got the error.
The error says: Script 'Game_Interpreter' line 1411: NoMethodError occurred. - undefined method 'refresh' for nil:NilClass