As stated above.
My setup is as following. I carry a SVD shouldered, and an AKS-74U with silencer in hands. When using the SVD, i keep the 74u in the backpack. As for ammo, i had 1 74 mag in the gun itself, and 1 spare 74 mag in a protector case, with 2 stacks of 30 rnds of 5.45 ammo.
After a brief firefight, I tried to top off the mag in my gun. I took the mag down into the backpack inventory, and tried to drag and drop the 30 rnd stack on it. Nothing happened.
After trying that for a bit more, i decided to right click/eject rounds from the mag (there were 14 remaining after the firefight). That did the trick, i was able to load the mag. It contained 30 rounds now. I tried attaching it back to the gun ("Add as attachment"), it didn't work. I thought about ejecting the chambered round that was left in the AKS-74U (maybe it was causing issues). Didn't help.
I took the gun in hand to see if i would get the "Reload the AK-74 mag" actionmenu prompt. Nope. I went back in the menu... and I saw the mag not there.
I thought to myself "Oh well, alpher and experimental..." and I wanted to load the spare mag in there. It was also gone from my protector case!
I thought it is just some visual bug or something and thought relogging might help. It didn't, no mag appeared anywhere.
I am playing with -newUI enabled.