Every single boss in the game has a specified list of set pieces that can be dropped. Only the final bosses of dungeons drop weapons. So, for curation, every boss in the dungeon but the final boss, will drop body pieces (unless they have a named item). Once you have collected all of the body pieces for that dungeon, your curation for those bosses is finished. You will continue to get body pieces you already have because that is the only thing those dungeon bosses can possibly drop. And you will have to kill the final boss as many times as weapons you have left to collect (or get lucky and drop those weapons in chests) to complete the sticker book for that dungeon.

The loot table for reference:
Mini bosses will drop either a hand, waist, or feet set piece.
Bosses will drop either a chest, shoulder, head or leg set piece.
The final Boss of a dungeon will drop weapons or jewelry.

This is the reason that you should be running dungeons on veteran (no simple chests on veteran) and always looking for treasure chests, as they can also drop weapons!

i dont usually aim for chests because chest loot is uncurated, so theres no guarantee you will get something you need and found its easier to speed the dungeons on normal for the boss drops that are curated lol

i personally have only been trying to fill out the collection book, and for speed/efficiency on that definitely just stick to normal for faster runs