I already submitted an in-game bug report, but not sure I phrased it in a way that makes sense. Plus, I can better describe it now.
Once you unlock the Collection, when you visit an appropriate jumping puzzle, you should get a Mysterious Grubby Package that drops BOTH a Wrecked Ornament and the desired Ornament toward the collection. Unfortunately, if you attempt to open the loot bag when your inventory is already full, you seem to only get the Wrecked Ornament, not the one for the Collection. The desired Ornament must be getting automatically deleted. You do NOT get the expected “You are Encumbered” screen. It just silently deletes the item that cannot fit into your inventory.
This caused a lot of confusion for me. Everything from thinking I was just getting incredibly bad drop rate luck, to even worrying I had missed some window of opportunity for completing this quest. I did about 5 jumping puzzles with my inventory full, and got the same results each time, only getting a Wrecked ornament. It finally took someone else pointing out that these Grubby Packages actually drop 2 items once the collection is available that I began to piece this all together.
The moment I made extra space in my inventory, the desired Collection Ornaments started dropping into my bags, in addition to the Wrecked ones.
While this quest might not be high on anyone’s radar, any kind of bug where legitimately acquired items are being silently deleted is NEVER a good thing. I think if you can just make sure these Grubby Packages follow the same “Encumbered” rules as any other loot bag that you open, this problem can be avoided.