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?