i put an ore stockpile close to where i was mining. it was on the same tile you can see the stockpile on the pic. the tiles i mined are highlighted. after mining those 2 tiles i noticed the stockpile had mysteriously disappeared.
i vaguely recall some bug with natural caverns opening up after mining a tile not behaving properly, back in world 8. is that what this was? (still, three worlds later?)