It seems similar to other bugs, in that the gun animation runs for dropping it, but instead of dropping it, the gun actually gets deleted.
It isn't where it was last dropped, it isn't at your feet, it's nowhere. this was newui and 0.59 stable, I've looked at other issues in the bug tracker but this specific issue where the gun actually gets wiped is not documented as being reproduceable.
Following steps (double carry + full invent + press key to pull out compass into hands) always deletes whatever gun was in your hands.
I've only done it three times, I dont double carry often. I'll attempt it a few more times to make sure i can reproduce it.
I had this happen a couple of days ago. I set my kitted akm down to drink ( I did not use hotbar) and when I was done drinking I went to pick my akm up, and it was nowhere to be found. I ran back quite aways where I came from hoping it might have warped behind me but never found it. It never reappeared where I dropped it like this problem used to do.