The UI will continue to notify you that the nemesis strongbox will be upgraded, even if you kill the mobs after finishing the last objective of the delve, but it does not actually upgrade the box after this point. I had 2/4 packs completed in Sidestreet Sluice but Brann kept glitching out and pulling too much from floor to floor. Knowing that previously (due to another Brann glitch) the box would update after completing the objective based on the notifications I received, I completed the boss since I was close to it and backtracked to kill the last two nemesis packs. But when I looted the final nemesis box, it was grey when hovering over it, with a bag of gold in it, and no crests. Either the UI should not indicate that the box is upgrading or the box should continue to upgrade as long as you kill the mobs before looting it. Also Brann is still terrible and this delve is incredibly claustrophobic. This was in an 11 Sidestreet Sluice during the “All that Glitters” story.