There's some interveinal chlorosis visible in the leaves as well. This means there is a mineral imbalance within the plant. Also plant looks drought affected, should not be growing in bare soil.

The top soil 4" is where the root reside and this area looks dry. You may want to maintain 2-4" of mulch year around to minimize soil moisture loss and protect the roots from temperature extremes and reduce yellowing a tad during the hot summer months. I would also acidify the oil a little more if you test it and it is uncomfortably alkaline.

PS - I shared your pictures with a doctor I know at Washington State University and who does work on rhododendrons and other plants. She said: "the rhododendron is planted too deeply. My guess is that it was planted without root washing and therefore was not planted at grade. What you are seeing is an impairment of the roots which manifests as yellowing and eventual dieback of the crown.

Please do not add any more chemicals to the system. I'd also advise a soil test so you can rule out nutrient toxicities which also seems likely.

In the fall (when dormant), you will need to dig up the rhododendron, wash off the roots, and replant so the root flare is at grade. No amendments to the soil. After planting, you will need a thick later of arborist wood chip (not bark) mulch for the health of the mycorrhizal ecosystem as well as that of the plant."