Cant fill my Ferraris gasoline beyond the gas tank limit, what kind of garbage is this Ferrari?
You literally have your answer in your own post.
You had a bunch of items on the ground, left the instance and when doing so the items get transfered to your lost items stash tab, if that tab is full which it was in this case, your items are probably gone or shows up after you have emptied the stash tab, not sure since this have never happened to me or anyone else i know of.
Manage your loot better and none will get lost.
Coming back to my first sentence, filling gasoline into a full gastank will make you lose gasoline, not gain. Same in this case but with your loot.