I went through the Gem Cave with the power of the Smiling Mask, but the cutscene doesn't occur on one save file. Same with the boss.
It's not supposed to I was just saying that if you want to get all the endings that's where the choice that determines your ending is decided. The game can be sped through very quickly for those looking to get to the ending although it's only a few different cutscenes there's no exclusive H-Scenes or anything unlocked by viewing all the endings