I'm wondering if the problem might be some kind of micronutrient or mineral deficiency?

Sun/heat/water stress can certainly lead to secondary insect or disease issues. And the patterning on the leaf surface does look a little like some nutrient issue. At this time of year there is really no need to treat as the shrub will be dropping its leaves before too much more time passes anyway.

@arbordave (SE MI) I designed a fast draining soil and it probably does have minimal nutrients. As Cotinus is not a heavy feeder I probably never did fertilize it.

@gardengal48 (PNW Z8/9) Just for kicks, I could try a minimal fertilizer feeding. I gather this might be more of a mineral imbalance than a nitrogen issue? I could give it one feeding of DynaGro, which is soluble and filled with many micronutrients. Then I could follow that up with a longer lasting balanced fertilizer with at least the key micronutrients added. Is there a tree-shrub balanced fertilizer that you like that is loaded with many micronutrients?

In most (all?) areas of the country, it is not recommended to fertilize after the end of July. You could do a dilute solution of the DynaGro now but wait till next spring to do any more. I use Osmocote Plus at that time to fertilize any woodies I grow in containers.