You plop down a Shulker Box in order to access it, but what happens is it can't be opened, and once you mine/destroy it with a Pickaxe it just disappears without you being able to re-grab it and all of it inside and the Shulker Box itself will be lost forever.
This seems to happen randomly and somewhat rare (still happened like 5-6 times the last couple days), and I haven't been able to find some kind of pattern. If you place another Shulker Box after getting one that is glitched it will get the same state and behave the same as well. Once all glitched Shulker Boxes are gone you can place them and use them normally (until it happens again).
It sucks that I can't replicate it. If I go into Creative and spam lots of Shulker Boxes it doesn't seem to ever happen. Maybe more people who have had the same bug can report what they think causes it.
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That's good to hear! Something I started doing was taking screenshots of the stuff that's in them, in case it happens.

This has happened to me a couple of times now, Windows 10 64 bit, version 1.13-pre2. The second time it happened, two shulker boxes were placed at the same time. Neither one would open and both were destroyed by a silk touch pickaxe.
Yeah, I wasn't aware that other people were having this issue. It's really a pain. It also still happens in pre2. Its happened to me twice at this point. ALSO, from my experience, this glitch only happened to me if a shulker box is renamed and/or recolored. I use shulker boxes a LOT and most of them haven't been renamed or recolored and I haven't had any problems with those ones. Only the customized ones seem to be breaking, could just be a coincidence. I'd appreciate if someone could confirm or deny this.
I just had it happen, it was an uncolored, unnamed, unmodified box. Last one placed. The other two broke and were collected normally. Placed on a grassblock, using a silk touch pickaxe to mine it when it disappeared.
I logged and relogged. I restarted my server. I mined all the grassblocks in the immediate area. Did not reappear..
Does anyone know if this affects 1.13 pre3?
I haven't had it happen yet in 1.13 pre3. I had it happen in pre2.
If it happens again I'll come report it immediately. Hopefully it's resolved unintentionally.
It is happening to me in pre3 as well, lost a few shulker boxes in snapshots and thought it might have been resolved in pre3 but it happened again.
It seems to happen in certain locations, and if you repeat it with another shulker box, it happens again every time. But blocks right beside it are fine.
Darn! So it still happens, huh. I did know about other shulker boxes getting the same glitch if you place more, I put this already in the main post.
Once you break all the faulty shulker boxes they work again normally until it happens again.
When you say it happens in certain locations, have you gotten the glitch to happen twice in the same area? As in, one shulker box not working then disappearing, then it works again, and then it's bad again?
So I think the issue is with the location not the box, I can put a box in other places and it works fine but in the are a couple of places in my base that if I put any shulker box there, they would stop working.
Example in my world (SEED "6939")
Location: -329 | 64 | 99
Another Example: -322 | 64 | 108
I do not know if the issue will be recreated if you make the world with this seed since I created the world in the last snapshot and upgraded to pre3 when it came out.

Can confirm in 1.13-pre4.
Can confirm in 1.13-pre5.
It seems that if you place a bed, then break it, then place a shulker box where the bed was, you can't open the shulker box and it will be deleted when you mine it. (This is in a survival world updated from 1.12.2.)
Now that you say this, I can confirm that in my world the places that I lost my shulkers where the places that I had a bed before.
I even checked a youtuber losing a shulker to this and he had a bed there before (https://youtu.be/Gv33PBiVRuE?t=6m43s)
ya it is reliable and easy to reproduce in places where there used to be a bed. it also happens with chests, except the chest still drops when mined.
I had that, with my precious items shulker box, and lost at lot of awesome stuff. luckily, I was smart enough to only play in pre1 in a copy of my world.