For those that don't open them it would not matter if none of the artcards was inside
They are meant to be opened to access the contents inside otherwise why should film arena even bother creating artcards or booklets etc
Most collectors will be opening these and lot would have got all of them
It's mostly only re-sellers who keep steelbooks sealed unopened or those that purchase to trade for other releases
Majority of customers that purchase steelbooks don't keep them sealed unopened they purchase to open and enjoy all the contents and is also the purpose of the release in the first place to be opened.
If a re-seller is stuck with a release with missing artcard
as far as I'm concerned as they are not meant to be re sold in the first place especially if got in the GB as listing on ebay is not allowed
Most of the film arena regular collectors would have been getting all of the X-Men FAC releases and will not end up with a missing artcard it's just a matter of switching them around