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.