I play the Breaking Point mod a lot (http://www.thezombieinfection.com). This issue is extremely prevalent as the game play of the mod relies heavily on storing weapons in crates/backpacks. I do not always see this bug, it's more of a risk of losing the attachments if you place a weapon in a crate or backpack. I do not typically lose the attachments when said weapon goes into the crate, but when I pull it out. Also, if the server restarts with said weapon still in the crate or backpack you will lose the attachment(s).
I do not have a mission to reproduce this, but I'll see if I can get a video of this happening.