The lava isle weird as it breaks the pattern established on the other isles in that you don’t have to solve the big island-wide puzzle to unlock the new ability there, you just gotta do the three almost boss battle screens to get the three keys needed to get to the isle’s rather useful upgrade. I understand why that is the case as the thing that would normally be responsible for that on that isle is… let’s say divisive, but it does throw some people off.
FWIW I was told that there is a subtle way that the game hints that there is something still to be found on a given screen, musical notes included, but I never noticed what that was myself.
EDIT: Oh no I killed the topic!!