Played the stress test #12 offline mode
If I put my makarov in it's standard pants slot (where it starts) and double clicked the mag it goes straight in to the gun. If I move the the gun to another spot it doesn't work. I made a little video of the interaction.
After spawn in
Take makarov to hands and load mag
Put makarov back in pants
Take makarov back to hands
Remove mag
Put makarov back to pants
Take mag to hands and double click.
It has a memory of of where it was in the inventory when it was in the makarov. If you move the gun to another slot like in a backpack and leave that slot open the mag to same slot and not to the gun.
Killed myself and tried on a fresh spawn and it work again.