It is just normal shedding of the older leaves. Chlorosis looks similar. It looks like this.
{{gwi:384546}}

If the younger leaves look OK, then it is not chlorosis. They are affected most.

How does the back of the leaf look?

Scintillation sun burns easily so it frequently looks ratty because of that. Also, I am not sure how it adapts to your Mediterranean climate. Here we get lots of rain in the summer months and it thrives. In drought years it has done well though.

Here is a link that might be useful: Rhododendron Basics by Harold Greer, Eugene, OR

The back of the leaf doesn't look particularly unusual. Just a few fungus spots that don't seem to be widespread on the plant. We've been having some fairly cool & cloudy pacific flow, but even on the sunny days it gets a decent amount of shade. The plant is in somewhat heavy soil, but that could probably be improved with organic mulch and a bit of gypsum. Thanks for the reply.

Regarding it being normal aging and falling of oldest leaves, I'd wonder about the chlorotic pattern. Gypsum works on alkaline clays only, which you will not have unless you have artificially altered the soil or live on an unusual soil for the region (you don't say what part of Oregon). Anyway, you should sample your soil and have it tested before monkeying with its chemical makeup. Depending on what you have it tested for, this exercise might perhaps also indicate something about the leaf patterns. Ask local Extension office for assistance with soil testing (you could also try cutting a representative branch or spread of leaves, showing the progression from normal to patterned and taking it to them).

It is not the classic chlorotic plattern. Most chlorotic leaves don't fall off and have a much better defined pattern. If the younger leaves don't have this patern, then it is definitely not chlorosis. If the yellowing leaves are only the older leaves and they are falling off, there is no mystery. It is just normal aging.

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Gypsum doesn't help the the kind of clay soils present in Oregon.