When I move items into my backpack, they disappear and I can't get them out again.

You can tell by the weight that there is something in the backpack. 

Greetings

  Hmm, yes, I've too encountered this bug before meaning that no; this isn't a bug that's prevalent from version 0.8.

I'm not truly sure what causes it exactly, last time it occurred to me, it was due to unloading items in quick succession, causing the game to delay and thus - glitch out.

  Unfortunately, if this glitch where to occur: There's no way to extract the items once inside the backpack. With the way you showed it, it's as if it occurs every single instance, is that the case?