Despite collecting the first series dawning armor on all 3 classes, it does not appear in my collections as owned/i cannot equip them as ornaments. I had noticed this issue in the collections with loot such as the faction rally gear and trials armor in forsaken too. (I'd taken a long break from destiny, roughly the release of warmind, til season of opulence). It appears that any items that were not in your inventory/vault at the release of forsaken(?) were voided from your history (i'd dismantled the new monarchy helmets, for example, and now cannot reaquire it from collections). If theres any way for me to reaquire these items, please let me know. Edit: made the name more fitting.
It was actually when Warmind launched, but yes, anything that was not in your inventory at that point (or since) was not tracked, and so was not added to your collection. Currently, there is no way to get those items back.