Ok this is actually annoying. If anyone doesn't know, after you beat Bowser's Fury and start going to the postgame content, the game will constantly force you into the Final Boss over and over. As far as I know there is no way to stop this. All I want is to 100% the game :/

Can't you cancel it out by collecting a shine?

There's this trigger where once the Final Boss is activated the sky turns red and when the sky turns red, any shine you collect won't cancel it out. It's odd.

Oh. That sucks. Though, aren't there a few shines that you can only collect when Final Boss mode is activated?

Just accidentally did it again, this time he went away. I don't know why he didn't go away the previous time. Seems inconsistent. Weird.