Bird litter has length of 4cm and the bag has max item length of 25.4cm (not sure where this is defined).
“pocket_data”: [ { “pocket_type”: “CONTAINER”, “max_contains_volume”: “6 L”, “max_contains_weight”: “10 kg”, “moves”: 400 } ],
There are very few items in general that can be inserted into a plastic bag. Why?
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.
The only difference between small cardboard boxes (which can accept litter) and plastic bags that I can see is that the former is rigid and has an explicitly defined maximum item length.
You read/type too fast. I just updated my earlier post. 
Wait a minute I’ve got another sentence to add.
I think that in most cases if a container is nested inside another container it should be automatically taken out and temporarily wielded or placed on the ground when you insert objects into it. It would then be placed back where it was, if it still fits. You don’t put bank notes into your wallet while it is still in your trouser pocket.
Yes, now I can insert…