Rhododendrons grow best in morning sun and afternoon shade. This looks like lacebug damage. They are a sucking insect and are usually an issue in sunny dry sites. 

At this point it is late in the season and no control is recommended. Begin monitoring next season in late April/early May and continuing throughout the summer. Lacebugs can have several generations a year. Look for the white stippling that begins on older leaves. Turn the leaves over and look for lacebug stages and black fecal spots. If you see this, you can spray a summer rate of horticultural oil on the leaf undersides where lacebugs are active.  You will have to monitor for this throughout the summer. Do not spray in high temperatures. Make sure the shrubs are watered during dry periods as lace bugs are attracted to stressed plants. https://extension.umd.edu/resource/azaleas-and-rhododendrons-identify-and-manage-problems

Marian