Returned with the patch. Even ticking/unticking or requipping doesnt always fix it and if it does, it's brief.
I think it is connected to getting into combat or mounts - I havent found the pattern yet, but its annoying to see it come back . It might even be connected to my shield since it happened when I switched to it as well from my rifle
What sleep is here? What dreams there are in the unctuous coiling of the snakes mortal shuffling. weapon in my hand. My hand the arcing deathblow at the end of all things. The horror. The horror. I embrace it. . .
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Just to make sure....this doesn't happen when you get near the Eternal ...um... something or other-shards that you use with the Special Ability key? I noticed that one gives you a backpack.
Nah it's an old bug that crops up anywhere in the game and removes your back from view. Was quite prevalent when mounts first appeared. Usually a relog gets rid of it, but that isn't happening now, it just comes back. It's unconnected to what you mentioned since I haven't encountered that.
What sleep is here? What dreams there are in the unctuous coiling of the snakes mortal shuffling. weapon in my hand. My hand the arcing deathblow at the end of all things. The horror. The horror. I embrace it. . .
OK, it is only occuring to me in the new map so far. I tried a core tyria map and no issue
The shield seems to be the thing. If I stow it and reveal whether through stowing, mount or glider - I lose my backpack visuals when I unstow in any way and sometimes my shield too
What sleep is here? What dreams there are in the unctuous coiling of the snakes mortal shuffling. weapon in my hand. My hand the arcing deathblow at the end of all things. The horror. The horror. I embrace it. . .
Still bugged. Would like my equipment to appear...
What sleep is here? What dreams there are in the unctuous coiling of the snakes mortal shuffling. weapon in my hand. My hand the arcing deathblow at the end of all things. The horror. The horror. I embrace it. . .
Five months later and this issue still hasn't been fixed.
I'd really appreciate the opportunity to see the item I spent my hard-earned money on...
its pretty bad inside of Grothmaw too, i never noticed it happen before but my ive had my backpack get turned off nearly every weapon swap or mount summon in that map.
Amana Silentchild; My Main
Ember Wandertooth; The Kingslayer, Kianda Redpaw; The Blazing Light
Why GW is Called Guildwars
Same happening in Dragonfall. Here's a video of the bug in action:

This is back or never fixed. Been happening to me a bunch in WvW
I second this - it's never been completely fixed. I, too, occasionally have the issue in WvW (been that way at least since gliding was added). And until I map hop or exit/re-enter WvW, the hocus-pocus persists.
From my experience, it's more prone to happen if your toon uses a shield and/or if you glide ever so briefly and then stop (within a second). It seems like the coding for making the determination of what item to show on the back cannot react fast enough. And once it gets confused, it's busted until a map hop or exit of WvW happens.
Anet,
One possible and acceptable fix to me would be that whenever a toon uses a Way Point in WvW -- on the same map or map hopping, kick off the current code that fires and resets everything graphically when map hopping. This would also fix the bug that sometimes happens when a Ranger uses GS3/Swoop and interrupts the animation himself -- you see the translucent eagle flapping on them continuously until they map hop and everything graphically gets reset. (As long as there are no other adverse effects, like excessively longer load times, this might be a decent band-aid.) Thanks!
Moonlight [THRU]
Back? I don't think it was ever gone. It's just a thing that can happen if you use a character that uses a shield.
Didnt need to type much and found this, well backitem disapears when using glider sometimes, worse in map like Dragonfall, all the time. (ad infinitum back btw)
Had that a few days ago with my necro in that new drizzlewood map. Were short befoe the north meta. My backpiece (Warbringer, that WvW legendary back piece) disappeared on drawing my weapons and stayed invisible until I unchecked and checked the backpiece visibility in the hero panel. But only once and then never again so far.
Praise Joko! And make necros great again!
Wow, I thought the back piece was never visible! For me, the show/hide checkbox in the hero equipment panel works only for the helm and the shoulder. Clicking on that box does nothing for the back piece, not even a pixel changes from whichever angle I look at it. So I thought it was never implemented!
Now I'm really curious to learn what I should be seeing at my back!
This bug has been an issue for years. It seems to be linked to gliding and mounts after being pulled or knocked down. The only fix is to switch maps an that’s not always a viable option.
Sorrow's Furnace Minstrel Invader
[DOA] Descendants Of Ascalon, [LOOT] Legendary Order Of Thieves
This is still an issue. One example would be riding on your mount in WvW, and when you dismount your backpack disappears. After that if you glide your backpack reappears. It might be related to the show / hide weapons, but not 100% sure on that.
I seem to only have this issue in Dragonfall. I have tried with multiple characters (of different races) with different back pieces.
I would fix this by zooming in (so my character isnt visible anymore) and then zooming back out, and my back piece would be there again. It's not much but would still like to see it get fixed.
I've noticed that capes dont disappear but this might be different for others.