Why Aren't Deltarune Chapter End Trophies Unlocking? Explained

So you've reached the end of a chapter inDeltarune, you've seen whatever nightmarish things have occurred back in Hometown, only to find that the Chapter End trophy hasn't unlocked. There's nothing to suggest that you've done anything wrong, and you've unlocked them for previous (or later) chapters. So what gives? Weirdly, it turns out that collecting an item in Deltarune can cause your chapter playthrough to not unlock the corresponding Chapter End trophy. Here's what causes the problem, and how you can fix it so you can add the trophy to your collection.