Gathering materials should go into your sacks even if your sacks are inside bags/backpacks.
Currently, if you have a sack in a large backpack and you have some wool in that sack, when you go sheer some more wool it won't stack on top of the existing wool in the large backpack - it just shows up in your main inventory.
This is also true even if the gathering mats aren't inside sacks.
In short, gathering materials should still stack up on top of each other even if they are in backpacks.
- Bryn and Jeffrey Miller like this