Please please let us know how they respond to more water, and what you conclude over time.
When the roots looked OK and the soil seemed moist, I was baffled. Was it moist down deep where the roots are, or only near the surface?
By any chance do the affected plants tend to lie in low spots in the soil, or high spots?
I assume you've grown in that bed before, and other plants did not respond with spots of sickness. Like someone dumped motor oil or persistent herbicide in a few spots.
Could someone like a kid or well-meaning visitor have "helped" by pouring excessive fertilizer or something else on a few plants?
How clean do you think the water you irrigate with, is? MAYBE if it were a little salty, and the salt accumulates non-uniformly, or different plants have different sensitivities, the unhappy plants have too much salinity. Seems unlikely, but if true, watering where the salt is strongest MIGHT spread that salt around to other plants.
One rather unlikely test you could do: in case there is some nutrient uptake problem like wrong pH or excess of some competing nutrient, you could foliar-spray some balanced fertilizer with micronutrients including chelated iron.
In the unlikely case where that helped dramatically, you might have some spotty soil problem that only manifests in a few spots. Then you could sample the soil where the symptoms are worst, and sample separately the soil where plants are doing best, and send both to be analyzed, with pictures of the damaged plants.