I just dug up the ailing phlox, and found what looks like a fungus. The first area I found was down in the root mass, but then when I looked closer I found a patch right next to one emerging stem. Meanwhile, the healthy plant a foot away was wafting its sweet scent into the air, and it was starting to sprinkle rain.
So I got in a kind of hurry. Couldn't wash off all the soil but inspected as closely as I could for more of whatever this is, and rinsed off as much of the root mass as I could without destroying it, filled the hole with plain organic potting soil from a bag, and replanted the phlox. I'll watch closely that its neighbor stays healthyand keep an eye out for more of this stuff. Which may or may not be the problem!
Anyone know what it is? Here are two shots of the little things at surface level, next to the plant stem:
and here is the little patch I found first, deeper in the root mass. My fingers for scale:
