Heatwave?

We had maybe one day above 90 all week.

Most of my young oaks are on their second or third flush now and pretty much look like yours. Lots of stuff effects them, foliage suckers, chewers, and diseases, and this growth looks bad. It happens to me most every year. During the heat of the day soft fast growth wilts, but recovers overnight, especially if you have lots of rain and clouds, followed by hot sun. I consider these June flushes a extra bonus of wood, adding to yearly height, but it isn't pretty. The old foliage on yours looks good, I wouldn't be concerned unless you begin to have problems with it.

I do seem to remember last year, the second flush on most of my oaks was kinda ugly (the first spring flush, and post-Aug 1 flushes, which a few trees had, were cleaner and healthier).

The Scarlet Oak growth is UGLY on the second flush, but at this stage, as long as it's functional and the tree grows, I'm good with it. Last year my young oaks flushed a third time around August 1, and that growth seemed to come after the majority of insect and fungal problems. There is the fall webworm and oakworm that hits oaks, but I haven't seen those here outside wooded areas yet.

A third flush has started. Looks MUCH healthier so far than flush #2.

agreed.
Thanks for posting the running updates. My bet is still on weather related problems.