Grats on that third recovery!
The fact there was a scroll of recall means the chest respawned, randomly, but the quest was still in the stage where the seal ring has never been taken yet.
I was wrong, when your inventory is full, you can’t pick up more items, they don’t go to the ground (that only happens when you are given an item, during a script, for instance a quest reward during conversation).
So my only plausible scenario, apart from a massive bug (which certainly is possible, there’s a LOT of stuff going on in the mere act of picking up an item here…), is that it was never taken, maybe you hit “take”, and didnt notice your inventory was full, and then closed the container window and teleported away. The next time you entered the dungeon, as you never took the ring, it respawned again on the chest, this time with a different random loot (i.e., the scroll). If it had been taken, it wouldn’t respawn again.