Please note that this is the Nintendo 3DS version, not the Nintendo Switch.
Okay, I beat the 8th District Boss (Dark Sun) a few months ago, but the medal scene didn't play at all nor did I get the respective medal itself when I beat it. Nowadays I'm starting to worry about how this may affect my journey to beating the game 100%. The yellow flag still does appear and indicates the status of the level correctly. But the "Extinguished" medal is not complete. I did check the Glitch page along with The Highlight Glitch which can correspond with this glitch (although I remember the entire footage in my head, so it wasn't done.)