I didnt found anything about this on the forum so here it go.
* this happens when you shift click your bag to unequip it while ur inventory has one grid of space lefting.
* you can also make your bag disappear if you unequip it while your "simple bag" is full.
* in order to recover it the only option is to craft another bag and equip it so you can move the old one.
* of course this can be used for extra 2 in simple bag while using a quiver too like the photo.