So this strange. LRM20 stored in torso with ammo in both arms. SRM stored in torso and ammo stored in one arm. arm with both 1 LRM ammo and 1 SRM ammo gets shot off. Other arm ok has LRM AMMO. The mech lost use of both weapons. This happened to me twice. I split the ammo so that if one arm got shot off I's still have ammo.

If you've used all the ammo from one arm and the other gets shot off you've got no ammo left.

That does seem likely. I was looking for a way to determine which ammo box is being used and tehre is none. There are a few hidden ways this game kinda games you.