What's going on beneath it? Looks like sod & big cracks in the earth? Lots of water doesn't help if flows right by the roots. The symptoms sure do look like drought!

Had you considered a top dressing of compost (bags at Big Box stores) and bark mulch?

Sorry should have mentioned that we have just put some sod down there from another part of our property that we built something on. We put it down after the plant had the problem though so they are not related at all.

It is experiencing some die back. Perhaps a small part of the roots experienced some damages causing the stems above to die but the other portion looks healthy. Cut off the dying part and allow the plant to build up the healthy part.

Continue to water but not overwater. Overwatering can kill it in as much as dry conditions can kill it. Just moisten and observe it especially during this heat wave.

Did you ever get an answer for this?

I have this same problem, only I have 5 plants. One has totally died now, and a second is half dead and half alive. It looks like it is moving down the line of plants. :(

Same thing here. I already lost more than 10 hydrangeas because of that and nobody seems to know what causes it :(