i seems i am not alone with this issues :-/. i had same probems in the last 4 months.
first i sent about 40 comics - they come back and 10 of them had this scuffs, smudge and dirt on the inner back of the slab - not on the comic - very easy and clearly visible because the back of the comics are black.
and one had a broken plastic part inside :-/
i returned them as ME. they come back today - and again, 5 of them the same, although i made a on each slab a note what was wrong and why i returned.
guess they do not care as much as they should. really annoying. in a few days the rest of 20 slabs will return, will then have a look. afraid i have to return some of them too.
honestly i will return them as long as i am happy with them.