My guess would be grubs. Dig up a square foot of patchy area and see if you see any. If you see a number of white grubs, that's your problem. You can put the bit you dug up back down. Water it in well. If it was out of the ground for a few hours, no problem.

Watering deeply every other day strikes me as excessive.

You shouldn’t have to water that much in our climate. We have chafer beetle here and their grubs do damage like this as noted above. You can apply nemotodes to combat them in July.