BM26 remains locked even though “Assault Expertise 1” is completed

I have fully completed the “Assault Expertise 1” challenge — it shows a check mark as completed in the in-game menu.
However, the associated BM26 gadget/weapon is still locked and cannot be equipped.
The item icon continues to display a padlock, as if the challenge were incomplete.

Restarting the game, platform, and refreshing the loadout did not resolve the issue.