How about ... You're a low nutrient freak and ironically enough while this may work for LPS and softies it doesn't work well for SPS. 'SPS like low nutrient levels' is one of those short often quoted hobby statements that confused the heck out of me when I was new to SPS.
Lights: blackbox LED - modded with 10x445nm, 6x420nm, 2x410nm, 2x395nm, lenses removed.running 50%B - 10%W ... any higher and I bleach stuff
Skimmer: bubble Magnus Nac6
Biopellets ~100ml
Doser for Ca, Alk, Mg
Carbon once a month (usually)
GFO monthly - has been removed for the past 2 months
Parameters:
Ca 450 ppm-- rock solid - salifert
Alk 7.8 DHK-- rock solid - Hanna
Mg 1380 ppm -- rock solid - salifert
PO4 0.02ppm - Hanna
NO3 3ppm
.02 PO4 is low, 7.8 KH is a little high but not bad. You are carbon dosing with biopellets so your skimmate stinks and the water has a higher bacterial load than normal. When SPS are added it's like being placed in a desert, they start begging for water. Keep the bottled water coming and they stay alive but don't look very good and get a bad sunburn. No water (food) and they croak. My latest horrible analogy.
The primary clue: Turning up the lights bleaches stuff. That is a sure sign of lack of available resources to handle the higher PAR.
I have two tanks, a 40 breeder with 5 small fish and a 150 with ... a decent number of fish. I have to feed reef roids to the silly 40 gallon every single morning AND afternoon or my SPS pale out. It's maddening. This also means I have to do more water changes or tolerate algae growing on the tank walls to make up for any lack of export. Small tanks drive me nuts. My 150 gets maybe 4 water changes a year, fed coral food 2 or 3 times a week, behaves completely different from the smaller tank.
I have had to move Blastos into my 40 because they start to show light stress in the big tank. If I stop feeding the SPS pale out but the little sensitive blastos do just fine. Sounds crazy right? I believe the answer is in the feeding habits. LPS can take advantage of few feedings by consuming a lot of food when available, SPS require a more constant smaller amount of food to stay healthy. In larger tanks, packed with well fed fish, food is relatively constant while in small tanks, which tend to have small fish and less of them, the water column is often devoid of food.
There's nothing quite like a big ole tang turd getting sucked up by an MP40 and spraying the water column with coral food.

Corals also prefer ammonia over other forms of nitrate so having a lot of fish tends to produce better colors, probably due to the higher (yet still very small) availability of ammonia.
If you follow any forums specializing in nano reefs you will note a common recipe for success with SPS is heavy feeding.
Lot's of probablys and maybes but in the end I think you'll find keeping KH at natural levels and feeding small regular amounts of food will help.