Thanks for the question

Interesting situation. I agree with the second arborist that it is not oak wilt. One of the most common and characteristic symptoms of oak wilt is that the top most branches are first affected. In the absence of your saying that this symptom is present, oak wilt is not a likely cause. I’m not completely convinced that this is heat scorch caused by high temperatures and intense sun. This is largely based upon the appearance of the leaves. I’m inclined to agree with the arborist who said it was a fungal disease.

The most likely fungal disease is Anthracnose. See:

https://www.gardeningknowhow.com/plant-problems/disease/anthracnose-disease.htm

https://extension.umn.edu/plant-diseases/anthracnose-trees-and-shrubs

Trying to diagnose oak diseases from just a few pictures is quite difficult. If you want to absolutely know what is affecting the oak, I suggest that you contact the Plant Disease Center of the University of Minnesota for a consultation. They may suggest that you send in some leaves for their analysis. They currently charge between $50 and $75 for an oak analysis. See:

https://pdc.umn.edu/

Finally, see the following for a good summary of the most common oak diseases:

https://extension.psu.edu/oak-diseases

Good Luck!!