User blog comment:Dragonjet/KC3改/@comment-731047-20150822043113/@comment-1820055-20150824032157

Right now we can only update the gauge when you visit the sortie page. So to update it right after damaging the boss, we do manual calculations and I suspect this is when you do overkills where enemy HP goes negative. I have not yet confirmed this though.

Though right now, going to the sortie page (map list only, no need to go to actual sortie) will update the gauges to correct values.

This is noted on GitHub