I've defeated the two monsters he asked me to. I was shown a cutscene in which he warns me not to be tempted and some other things. After that, I'm shown an explanation video for the Wasp glyph. Thing is, I already had the Wasp glyph. I read somewhere that Montgolfier gives me a heart, was this supposed to be it? Did the game glitch on me or is this heart reward something I'll get later?

Hmm. I can't recall that section.

I suspect that the game glitched and for some reason showed you the Wasp video, but did not actually give you a new heart. You should still be able to get all 21 hearts.

Montgolfier does give you a heart, but only at the very end of the game. It is ALWAYS the Breakthrough heart for a reason you will understand when you get there. I won't say more or I'd spoil it.