I was plagued with a similar issue in my tank for an extended period (1-2 years). At the time, my nitrates were super low, reading about 0.2 ppm on the Salifert test kit. Any attempt to raise nitrates would unleash STN with a particular pattern. At the time, I suspected a coral pathogenic microbe was the culprit. I wanted to do a biome test to see if any of the known pathogens were present, but the tests were not available in Canada at that point. So I never did the test.
I first noticed the problem affecting cyphastrea corals. I would get individual polyps turning white, bleach, die, and then the tissue surrounding the dead polyps would recede and die. It affected all SPS and LPS that had smaller polyps. LPS with big fleshy polyps were unaffected. Occasionally, a coral would survive when a small patch of tissue would make it through the STN, sometimes only a single polyp, and would grow back. However the new, re-grown tissue, was not immune or resistant to the issue and could, and would, be affected by future occurrences. The issue appeared to affect encrusting and plating corals more severely than corals with a vertical growth pattern. At the time, I wondered if the disease was causing affected corals to loose their tolerance for higher PAR, which would explain why corals with horizontal growth patterns appeared to be affected the most.
Any attempt to dose any of the commercial nitrate or DIY additives to raise nitrate, would trigger an outbreak. It was a very frustrating time. The issue eventually went away on it's own. Though I did take steps that may or may not have helped to resolve it. I started dosing Dr. Tim's Eco-Balance as one of it's stated features is that it contains bacteria intended to combat vibrio. Not knowing what pathogen was causing the issue, but knowing that vibrio can cause coral mortality, I decided to give it a shot. I used the Coral Colours recipe that they provide as away of adding the Eco-Balance on a regular basis.
Months after the problem receded, I eventually started dosing ammonium and that did not spur the problem to re-appear. However the issue may have already resolved by then anyway.
My tank is currently 1.7 ppm nitrate as of today, and has been as high as 15 ppm recently, and the problem is not occurring. Though whenever I see any STN in the centre of a coral, away from an edge, I start to worry that the unseen coral killer is still on the prowl.
I continue to dose a weekly addition of either PNS Pro BIO or FM Rebiotic.