Need help with a rhododendron problem
I am sure that you have eliminated this as a possibility.
And, it sure looks like sun burn to me.
Is there any chance that there is moth moisture and full sun?
We sometimes see a burning associated with full sun and water on the leaves with the shade loving varieties of rhododendron. There are sun tolerant varieties of rhododendron for sunny places in your yard.
question - was there once shade on this plant in the past? Has a tree or some other sun barrier been removed in the past couple of years?