This hasn't happened to me before. But when I started farming building resources a few days ago, I started to have this noticed a few times. When ever I end a mission I started with 0 stone, wood and metal it appears as if after that mission my wood bricks and metal count as occupying item slots but after that mission its just fine again. I couldn't yet find a pattern so it might have other causes.
So since recycling is just dropping items in world and not possible in the menu if backpack would overfill and also any rewards go to the storage and rather overfill the storage then your backpack.
Is it known how the backpack could overfill?
Best Answer
At the end of a mission, you are given certain items as rewards. These items are placed in your backpack if your storm shield storage is not opened yet. When your backpack is full at the end of a mission, the items are put in as overflow. You cannot access them until room is made. Your backpack does NOT count the wood/metal/stone as spaces, since these are automatic. Once you open your storage, you can move these items there, so you can build a stockpile, as having extra is extremely important once you get higher.
One suggestion, is to recycle items you don't need. IE: low level guns. As far as traps, I would HIGHLY suggest using them in your base, even low level ones can help kill husks. Don't place patrol wards or campfires as they are timed, and will quickly get used up. They don't stay past the actual mission.