TL;DR: In Patch 3 backpacks/pouches can only be filled up to their initial 20 slots and can't hold more stuff. The slider doesn't work!
Not sure if this is a bug or actually intended but here goes. During my previous playthrough I always picked up extra backpacks/pouches that I found along the way and stuffed those with items I planned on selling to a vendor or to have them in my regular inventory so I could keep potions or scrolls in one tidy slot (I especially loved pouches for my wizard!!). That was a very convenient way to keep my inventory free of clutter and you could even sell the entire bag with contents directly to a vendor.
However, now that I started a new game with Patch 3 it seems that something happened with the backpacks as I can no longer fill them up. I can put items in the 20 visible slots but I can't put any more items in. The little slider on the right doesn't work and won't let me pull it down to make more slots available (see image).
I really hope this is just a bug and not something that Larian intended as I was really fond of putting all my vendor items in backpacks as well as carry my extra scrolls/potions/keys in pouches. Now it seems you can only use the 20 available slots and no more.