Star Dawg in 3 gallon felt pots, running peters excel cal-mag, ppm has been around 500-550, Ph’d at 6.8.
It appears that you are just on the edge of a Magnesium issue. Likely caused by your CalMag additive. Too much of something will cause problems with other things. In this case, it appears that the abundance of calcium has blocked (to some extent) Magnesium uptake. If you have been using CalMag for every watering, you should have more than enough in the medium for quite some time. Try not using it for a couple of weeks and see if things improve.