I don’t know if this is the case for plastic bags, but pockets can have specific items they are allowed to contain. Travel Wallets for example can’t hold cash. HOWEVER you quoted the pocket data and it doesn’t have any restrictions shown.
Try wielding your plastic bag then inserting stuff into it again. If you have your plastic bag crumpled up and stuffed into a shirt pocket (or whatever) then not only does the thing going in have to be small enough to fit in the bag - it also has to be small enough to fit in the shirt pocket. At present the UI lacks any sort of explanation / warning if this restriction applies.