I'm alarmed to discover from the comments that the bonus challenge rewards a different ending, because I would have wanted to achieve the normal ending first, and started the bonus challenge on the understanding that there really was no reward (as was stated at the start). Now I'm partway through the bonus room and I really don't want to lose that progress, so (I assume) I'll never see the normal ending unless I lose the progress or replay the whole game.
Other than that, phenomenal as always. Does feel a bit too linear to be called a "metroidvania," the same way Amidst the Sky did; even though you're technically backtracking for collectibles, you generally know exactly where to go and it's very driven toward progressing in those few directions. (I realize now it wasn't you who called it that, actually it was Bart Bonte. But the point stands.)