- found ammo stack was unpickable (no Take option in context menu, despite having empty space in backpack);
- tried draggin it to backpack but it failed to move;
- split ammo option was available, so i started to do so, repeatedly;
- as i had subsequent (splitted) portions of ammo, the first slot in empty backpack was unusable, despite looking empty;
- as the split ammo option disappeared from ammo stack on the ground (which was still showing the initial amount of 18) i moved away a bit and dropped backpack to the ground;
- this revealed 18 ammo in the first (seemingly empty) slot, plus all the ammo i picked up by splitting - i was able to combine all of it, ending up with 36.