Have you checked your runoff ph? I think you are correct, you have a magnesium deficiency, the question is why. The coco website says that it is pre-buffered, so presumably that is not the problem. Since you are using reasonable amounts of cal mag, my suspicion is that you have some sort of lockout going on, either from pH or from other nute concentrations. However, I am too new at this to help you with details on interactions with other nutes.
If I were you, I would double check the ph of runoff. The way I do it with autopots is to turn off the nute supply and either empty the reservoir by sucking out the liquid or just letting the plant drink it. Then I water from the top with enough nute mix to get the reservoir full, and check the ph and EC in the reservoir. If the pH and EC are good, then you know that your coco has not crept out of range, so the problem is either a simple lack of magnesium, or some sort of lockout caused by other nutes. If the ph is not good, I flush the pot with multiple gallons of pH'ed nute mix at the EC that I want the plant to grow with. Do not flush with just water, the change in EC is an unnecessary shock to the plant. Flush with exactly what you want to feed with until the runoff is the same as the input. Then you know that the coco is where you want it to be, at least until it drifts again, if it does. On the few recent occasions when I have done this, I have just kept the runoff nutes, and re-used them once the ph and EC are adjusted back to where I want them.
If your coco pH is good, you are going to need advice from someone with more experience -
@Waira would be a good bet. Having said that, I pH'ed to ~6.0 in my coco grows, so maybe 5.8 might be a bit low.
Good luck with it.