well, that is a crispy looking plant.
I would second digging it up and seeing what's going on underneath. Sometimes one can get a nematode or something else and the other not, or one can have something under the soil disturbing its existence, and the other not.
I had a spot in one of my beds where everything seemed to die. Then this summer while digging in there I noticed some green plastic several inches down. Turns out the previous owner some time ago had put green plastic garbage bags down along the side of that bed (I assume to stop the weeds from under the porch from encroaching on the garden bed). It was effective at stopping the weeds, but until I found it and ripped it out, nothing would grow in that spot! Not that that's what's causing your phlox to wither!
