I received an in-game mail from another character on my account. In it were some gray items, including an Ice-Bound Plate Gloves and a Sentinel Musket. These items are logged in chat as being received and briefly landed in my bags, but quickly vanished. This was not an accidental deletion by me, but the game itself deleting items from my bag. I took screenshots of the chat log and my bags showing the empty spaces where the items should have been. I am not simply seeking item restoration here, but asking what happened to cause the items to be deleted and how do I avoid it in the future? As someone who lost items from multiple guild banks, I am deeply disturbed that it seems to be happening to bags.
EDIT: I opened another in-game mail and moved its items into my bags. The spaces where the missing items should have been were not filled, implying the game knows there is something there, although I cannot interact with it. Exiting the game and logging back in made the items re-appear in my bag and I can interact with them normally. While my immediate issue is resolved, I would still like to know how this happened at all.