I was just going to log off and i changed my equipment to disconect with the bow and quiver when i noticed that when i take out my backpack with only a vertical line of inventory slots to the right of my inventory the backpack literally enter in 2 vertical slots of the inventory cutting the backpack drawing. But it happens when you use shift+left click on the backpack when its in your equipment and when your inventory is full and only have a vertical line in the right. Here is a pick:

I hope it helps to fix it.