If you do treat, bear in mind :
A) you can easily spread infection on the ointment tube, your hands and clothing, etc
B) bringing them together will increase the spread of infection all by itself
I hesitate to contradict a vet, but in my experience, they pretty much always get better (with total recovery of sight) with or without treatment, and the two factors above can make it a longer process with.
Personally I only treat if I think it’s a welfare issue; and then would try to isolate that sheep without bringing them all together. And disinfect
everything after handling it.
It’s so common to have this in sheep newly arriving somewhere that I suspect a stress component.
Of course don’t mix them with the existing flock until they’ve been two weeks clear of symptoms.