Water meter devices might not be giving accurate readings, especially when some plants are more sensitive to excess moisture or drought than others. Feeling the moisture levels about five or so inches deep is the best way to determine when they need water. Rhododendron prefer some consistent moisture without prolonged sogginess, nor do they tolerate drying out. (Root rot is more common when grown too wet, as they are very vulnerable to that.)

https://extension.umd.edu/resource/phytopthora-root-rot-shrubs

https://extension.umd.edu/resource/diagnostic-chart-azalea-and-rhododendron-problems

Container-grown rhododendron tend to become quite pot-bound at nurseries because they naturally grow very shallow roots that have limited space to spread; it's not uncommon for them to not even fill up the container to its full depth. If a matted root ball isn't sufficiently loosened upon planting to encourage roots to spread out, the core of that root mass can dry out much faster than the surrounding soil, especially if the probe isn't measuring the interior of the root mass itself. Therefore, even if the probe is reading levels accurately, it may not be reflecting the true moisture level of the soil within immediate reach of the roots. If the nearby rhododendron are receiving the same care with regards to watering, and the drainage in their planting site seems to be the same, then an issue with the root ball on this individual plant is suspected.

Is the rhododendron in full sun? The blooming hydrangea behind it suggests it is receiving a lot of direct afternoon sun (which would be needed to keep the hydrangea blooming well). If so, consider relocating it to a site with afternoon shade (dappled or otherwise). Full sun can stress rhododendrons as they are adapted to woodland conditions; for example, a canker disease can kill branches when plants experience heat and drought stress.

Make sure the mulch isn't touching the stem base(s), as this may encourage disease or wood-boring insects like rhododendron borer. See if the bark exhibits any cracking, holes, or fine sawdust residue.

All you can do for now is to keep monitoring the plant for watering needs and, should any individual stems die back, trim them out.
https://extension.umd.edu/resource/watering-trees-and-shrubs

Miri