Memorable Missions - Open all the Strongboxes - Not possible to complete

This subgoal of this challenge is not possible to complete. Clearing all the strong boxes does not reward completion

Just ran into this also. "Open all the Strongboxes" is not completable in the Challenges.
Reference #: 338,904,897

Two things - you need to do Kirac map with enchantment of "It's a lot of boxes in here" or smth, And you need to kill a boss of the map. Kirac's mission completion procs on boss death, i'm not even sure you need to open them boxes.

Edit: Yep, just checked it with "Complete the excursions" one - blitzed through to boss ignoring everything, killed boss and it gave completion and challenge piece