I had some full stacks of pristine 5.56mm ammo in the attachable pouches of a ballistic vest. Holding a pristine, empty STANAG in my hand, dragging an ammo stack from the pouches to the magazine in my hand did not produce an interaction - the square turned red, rather than yellow, and I was unable to load ammo into the magazine. I moved an ammo stack to another inventory slot outside of my vest pouches (my pants, I think). Then I was able to drag and drop ammo from that slot onto the magazine, producing the normal interaction (yellow outline, option to load mag, successful mag reload). Tested only by accident using 5.56 ammo and a STANAG. All items were pristine.
It appears ammo in the ballistics vest pouches may not be able to be loaded into a mag/weapon.
This also remained the case when I tried a hotbar reload, i.e. ammo stored in the vest pouches was hotbar-linked as item 3, and the empty mag was item 2. Pressing 2 then holding 3 should commence loading the mag, but this did not work occur when the ammo was kept in the vest pouches.