Was told about that problem a little while ago and I think it was a bug where the menu wasnt reading the attachment tables correctly for GL. So the attachment stat for that weapon gets incorrectly set as tho it hasnt been unlocked. IMHO they did things for attachments in a screwy way for some things but can only work around it. So far the problem hasnt happened in the next release. For now, delete the player profile for the mod. Ranks are reset of course but it wont hurt anything otherwise.
In the far future, if time permits, we would like to reintroduce weapon and perk unlocks and maybe a slightly more elaborate ranking system. But I fear opening a proverbial can of worms.