sometimes maybe, but in my case it disappeared, and this happend already multiple times: once i was on a street, the other time i was on a open field with no grass whatsoever.

have this issue too

Doubleclicking on the hand guard while having it on your back puts the hand guard on your hands/inventory making it possible to then add a hand guard to the weapon again

nop this doesnt work, its not that you could even swap the attachment, you cannot even detatch it: its like a non moveable part, and when you try to drag a new attachment of same class over it the item you are dragging is getting this red - "is not fitting there" background color.

then handguard gets lost if you paint it and have full inventory btw.
EDIT: sry, i ment the buttstock. but it appears that it is just not visible, it's actually still there.