The other backpacks (Fae, Vulpera) hide the staff. Some might like this as a feature, but I’d actually not mind clipping, since the game has plenty of it anyway, but staff in backpack somehow doesn’t seem jarring to me. And if Blizzard is OK with hiding a staff, then they could just as well make it a transmog feature or such, as requested by some people.
Although it’s a double bummer, because the Fae backpack I farmed quite a quite for to have the perfect color scheme for my mage … well, on vulpera there is that head-body misalignment, so like some other non-cloak back transmogs, too, it looks bad that way, in addition to the staff not showing even though I really want it to.
This doesn’t just concern backpacks, of course, but any non-cloak back transmog. They all hide the back-sheathed weapon.