This is something that has happened to me two times now. On my current hardcore run, I found a green spray can. When I disconnected and reconnected, the contents of the can was empty. It was 100% full when I had picked it up.
Later on after trashing the empty can, I found another one, this time it was pristine and 100% full too. I reconnected to the server only to find that it was now empty for no reason. I tried to spray it on my Mosin that I had just found, thinking that it was just a bug. Yet, it said that it was empty.