In multiplayer, players are unable to directly reload STANAG ammunition on their SDAR's if they have UW ammunition loaded. However if they first dispose of every UW ammo clip, then it is possible to load STANAG.
Trying to reload STANAG actually removes the magazines from the players inventory but it is not loaded nor seem to be dropped to ground either.
Note that the multiplayer host (if applicable) is not affected by this problem. {F17218} {F17219} {F17220} {F17221}
Tested by multiple players on different missions.
Yeah it seems like some kind of server<>client sync issue... basically if you're the server (player hosted) you have everything just fine (camo, gears, ammo) but if you're the client joining the server you can't use camo/gear switching and have this kind of ammo problem
/upvoted
Was about to report the same thing. Uploaded 4 screenshots (s1-4.png).
s1-s2: having one stanag mag and one dual purpose allows to switch between them by right clicking on the magazine loaded into the weapon
s3-s4: after adding one extra dual purpose mag and after switching from dual purpose to stanag cannot switch back to stanag by right-clicking on the magazine loaded into the weapon
Tests done in multiplayer editor