I agree. It is some sort of interaction between watering/feeding and sunshine/temperature.
The exact same thing happened to a single plant of mine this summer. The rest growing around it were fine, so there was also possibly a genetic component to this.
Its a stress response and it disappeared when the weather turned cooler,
so any or all of these could help.
lower temperatures/less sun
more watering. Wet growing phrags should never dry out. It’s impossible to overwater them. I sit mine in water year round.
You should play around with these factors until you find a combination that works for this plant.
My phrags are enjoying the potting mix that Ray described. Gro-dan and large sponge rock, half and half.
I would urge you to try that for your phrags. Of course, use the K-lite fertilizer with ro water if you have it.
Correct potting solves many problems.
Thank you for all your help. I live in Western Canada and unfortunately, don't have access to First Rays. Any other fertilizer recommendations? My water here is very soft. . Maybe Im not leaching enough. I will look into the gro dan and sponge rock.