This is a bit hard to tell from this picture, so I have some follow-up questions:

Is this dieback happening on all of the plant or only on some branches?

Have you noticed this type of damage before and how old is the plant?

Are there other plants being affected as well and is it possible that herbicide was applied in the vicinity? I see some other plants in the background that also look to be affected.

What are your watering practices, do you use irrigation in your garden and if so what type and what time of day?

Please send us some additional information and pictures if you can. If we still cannot make a determination, we may recommend that you send in a sample to the MSU Plant and Pest Diagnostic Lab.

Thank you for the additional information and pictures. They are very helpful. I don't think your fungicide spray would have caused this as long as you were applying in accordance with the label.

The symptoms don't exactly match Botryosphaeria canker, since I would expect the damage to follow some sort of injury or pruning. However, that is a common disease rhododendrons can get. https://extension.umd.edu/resource/botryosphaeria-canker-shrubs/

What seems more likely is that it is Phytophthora dieback, which is fairly serious and could result in the death of your plant. Typically, we see stunted growth and leaf yellowing and necrosis first, though, not sudden dieback of the growing points of branches. https://extension.umd.edu/resource/phytophthora-root-rot-shrubs/

It will be important to know which of these is occurring (or neither, maybe it's something else I haven't thought of). 

More reading on Rhodies, a great resource from Univ. of Maryland Extension:https://extension.umd.edu/resource/azaleas-and-rhododendrons-identify-and-manage-problems/

With the Botryosphaeria canker, you can manage the disease and prevent further infection. With Phytophthora, there is really no cure besides pulling the plants. I would recommend sending a tissue sample of tissue displaying symptoms, healthy-looking tissue, and some root tissue in to the plant and pest diagnostic lab to figure out for sure what the issue is. 

https://canr.msu.edu/pestid/submit-samples/

You'll want #1 the Plant Health Analysis.

Here's the link to the submittal form: https://www.canr.msu.edu/pestid/uploads/files/General_Sample_Submission_Form_May2022.pdf

That is concerning that's it's spreading to other plants. I'm still not able to say definitively that it's due to the Phytophthora, though, and not something like herbicide drift. That's where the lab analysis would be helpful.

It's unusual that you've had the plants growing well for 10 years and this suddenly happens, which is why I'm still not sure this isn't due to some sudden event. I think that if you'd like to save the plants or to continue growing rhododendrons there, it's worth it to submit the sample to know for sure.

If the sample comes back positive for Phytophthora, then you'd want to pull those plants since their condition will just continue to decline. Before introducing new plants, you'll want to make these adjustments with how you manage the site.

• Avoid overhead irrigation which can splash the fungus from the soil onto leaves
• make sure your site is draining properly, low areas that collect water runoff and locations near downspouts are not ideal locations for rhodies. Construction of raised beds or grade changes may be needed to ensure proper drainage
• Resistant varieties are available. However, if they are flooded for 48 hours or longer, or are drought stressed to the point of wilting, resistance is temporarily lost and the fungus can invade.
• Symptoms on older plants can be caused by introducing infected plant material into the same planting bed, changes in water drainage patterns, and low plant vigor

If the analysis is negative for Phytopthera, then the damage could be due to something else that does not warrant removing the plants and you could simply see if they recover.

I hope that helps with making a decision about what to do!