This looks like a fungal infection of some type, possibly gray blight.   Recommend you remove and destroy infected leaves on the shrub.  You should also clean up dead leaves beneath the rhododendron so spores will have one less place to overwinter and then possibly reinfect the shrub in the spring.

Here is some information about gray blight on rhododendrons - https://hortsense.cahnrs.wsu.edu/fact-sheet/rhododendron-gray-blight/#:~:text=Gray%20blight%20is%20a%20fungal,found%20on%20the%20leaf%20surface.

The only way to positively identify the disease would be to submit a plant sample for diagnosis.  The Oregon State Plant Clinic conducts this disease diagnosis (for a fee).   Here is a link to their site - https://bpp.oregonstate.edu/plant-clinic/submit-sample/services-and-fees

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