Good description with plenty of details. However that is a mystery situation as your plants seem to be getting good care and growing conditions. I suspect a disease but it could be that the soil is staying wetter than you think or perhaps if the watering is by sprinkler the foliage/stems are being kept wet for too much of the time thus inviting disease problems. You might try some general purpose fungicide sprays as a protective/preventative measure. Something with triforine or chlorothalonil as the active ingredient would be a good choice. Prune out any dead/diseased shoots prior to spraying. Dip your pruners in rubbing alcohol or spray with Lysol after cutting a diseased shoot and before cutting another plant.
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