I could be wrong here but from what I understand is that flowers brown when there isnt enough water, the plant will sacrafice the flowers. Hydranges love water, try watering more. I usually deadhead mine so the energy goes into more blooms.

I don't think any mophead blooms are still looking fresh this late in the season. You may get a month out of them but they typically fade/brown by August.

Things like drought, sun, and soil condition can affect how long the blooms last, but they usually peak for 2-4 weeks before fading.