also experienced this issue. won a couple shards recently (probably a day or two ago) over a somewhat lengthy session, one of them from platinum spins, one of them from regular spin i upgraded. cards no longer exist.
its almost as if there was a rollback at some point we may not have been told about. certainly those other things are possible generally (the not leaving arena, switching computers, etc), but i suspect the recent pvp2 bug involving firefox users (it crashes your browser basically atm) somehow is more directly responsible. also, i dont change computers so its definitely not that in my case. also moved around elements menus before exiting session, so not likely either.