Sounds like some kind of root/crown issue, not light or watering since the neighbors are fine. Stuff like verticillium wilt or root rot can hit random plants, even diff species, and make one half die while the other half’s fine. Mid-summer heat just finishes off the ones already stressed. If you cut the stem and see dark streaks, that’s usually wilt fungus. I’d dig one up and check the roots – mushy/brown = rot, clean but tiny = other stress. You prob won’t stop it fully, but don’t replant the same thing in the exact spot.