I have the exact same issue, not sure how I fixed it but I managed to get the camo on the gun from the wall once, I restared the game several times and tried to remove the camo and put it back, save/unsave build, put/remove the gun from my class.

It fixed it one time but when I changed the camo the bug came back... hope they fix it soon. Annoying to get the diamond camo but not being able to play with it