Unable to equip newly unlocked attachment while in a match.

While I'm in a match and I unlock new attachments for a gun, I often find that I can't apply those attachments (or any attachments really) while I'm still in the match. 

I asked some other players about it and they confirmed that they get the same bug as well/

This bug was also in the Beta, but I forgot to report it then so... my b 😅.

I have attached a video of it.

In order to reproduce you have to level up the weapon's mastery and then in the deploy screen try to equip any attachment.