I’m not sure if this happens with all sheathed weapons, but it seems to happen with all weapons that I have tried that mount on the back. When transmogging into Dark Ranger General’s Kit, my character’s bow is visible as expected. But if I change zones in any way (i.e., go through portal, enter dungeon, etc.) her bow disappears from her back. The only way to get it back is to transmog into a completely different outfit, which causes the bow to appear again, and then transmog back into Dark Ranger General’s Kit. Then her bow is visible again until the next time I enter someplace that requires a loading screen.
EDIT: I should add that when you transmog to another outfit, you have to unsheath and resheath your weapon again to make it appear. Then transmog into Dark Ranger General’s Kit to have it appear on that outfit again (until you hit another loading screen, of course).
EDIT2: Actually, I just dismissed a pet, and it caused my bow to disappear. So, it seems to be happening due to a number of different factors.
EDIT3: I just realized that there are 4 chest pieces associated with this set. I was experimenting with just the default one. The two that have a quiver have this issue. The two that do not have a quiver seem to be fine.