The English Ensemble Stars Wiki talk:About & Rules/@comment-33537740-20171101131928/@comment-29535521-20171101151015

Nah, it's not that dumb!

Every gacha that gets paired with an event gives cards that, for that event only, raises stats (x2 maybe? Not entirely sure but a significant amount) and increases points gained when used to finish a live.

The 4*s that were recently given as daily rewards also got that boost which actually lasted for longer than a normal event, so my guess is that they went back to their normal 4* stats (which should still be p strong if you maxed them!) resulting in your overall team's power going down. I hope that helps!