Your ROS doesn't bloom until the second week of August?
Mine blooms after the 4th of July.
That is strange.
If your ROS is blooming, and is still coming back every year, it is probably just the soil mabey.
The spots aren't killing the bush, so just try a couple of different things and see if they improve.
You will have to try next year though, it is late in the year to try and figure it out.
Try investigating sun exposure. Do they get enough?
What are they planted next to? Could that be effecting them?
Try not watering more than once every 2 weeks, try fertilizing them if you haven't already, or not fertizing if you are.
Try different mulch next year, or not using mulch.
You get the picture.
No, they aren't extremely suscepible to any diseases at all.
They prefer acid soil, so mabey you could check to see if your soil is extremely alkaline.
Rose of Sharon does from time to time have a few yellow leaves which is normal. Alot of them and loosing leaves in August isn't normal.
It doesn't sound as if your bush is in extreme danger anyway.
You will see what it is.
Good Luck!