So here's my thought -
If I'm doing my math right, 4000ppm on the overall "chloride ion" test (because the K-1766 is measuring chloride) is equivalent to about 300 lbs of "salt" in your pool and you roughly have 40% of that as magnesium chloride. If you do all the molar mass conversions correctly (and again, I may be off since I'm doing this literally on a napkin), your adding about 1680ppm worth of detectable mineral hardness to your pool water since the CH indicator dye is sensitive to both magnesium and calcium ions. If your tap water (and thus fill water) is as low as you state in calcium hardness (75ppm), then the vast majority of mineral hardness in your water is from the magnesium in your salt, or about 22 times as much magnesium relative to calcium.
So why isn't the CH test working? Thanks for asking, that's a great question ....
The CH test relies on an indicator dye that complexes with Ca2+ ions and changes color (red when calcium is present, blue when there is no calcium). It then uses a stronger complexing agent, EDTA, to strip the calcium away from the dye thus turning the red solution to blue. The concentration of the EDTA titrant (R-0012) is set so that each 40uL drop removes a certain amount fo Ca2+ ions.
Now, there's always a devil in the details. Magnesium ions (Mg2+) look almost exactly the same as calcium ions to the indicator dye and to the titrant. In fact, thermodynamically, there is very little difference. So the indicator/titrant is insensitive to calcium or magnesium and thus magnesium can act as a positive interference. If half of your ionic concentration is from calcium and the other half magnesium, then the indicator dye will react the same.
So how do we tell the difference between calcium hardness and magnesium hardness? Great question again, thanks for asking...
Magnesium is a very soluble ion...except at high pH. Seawater contains lots of sodium but also lots of magnesium (it's the second most abundant element in seawater). Milk of Magnesia (the stuff you drink when you have acid reflux) was discovered by adding lime water (Ca(OH)2) to sea water causing the precipitation of Brucite, or magnesium hydroxide (Mg(OH)2). At a pH below 12.4, calcium compounds are still fairly soluble (unless lots of carbonate is around) and so you can selectively precipitate magnesium before you precipitate calcium. This is the entire basis of the lime softening process that is used to soften hard water.
So, in the Taylor test, there is a chemical called the "Calcium Buffer" (R-0010)....it's kind of misnomer, but it is nothing more than sodium hydroxide, or lye. The sodium hydroxide is used to raise the pH of the sample water and add hydroxide ions (OH-) so that magnesium will precipitate out of solution according to the following reaction -
Mg2+ (aq) + 2 OH- (aq) ----> Mg(OH)2 (solid)
So the R-0010 is performing two functions - (1) it is adding hydroxide ions to the water which react with magnesium to form insoluble Brucite, and (2) it is raising the pH of the resulting solution. One thing I did not mention about the calcium hardness test is that it works correctly when the pH is above 10.1 but below 12.4 in water that contains carbonate. The reason being is that the chelation of the metal ions is more rapid at higher pH but you want to stay below a pH of 12.4 so as to not accidentally precipitate calcium carbonate.
With all that said, because your water is so heavily laden with magnesium, the R-0010 simply can not add enough OH- in 10 or 20 drops to sufficiently precipitate all the magnesium and, because the magnesium is using up all the added hydroxide, the pH of the resulting solution after 10 or 20 drops of R-0010 is simply not high enough. Your water is so far out of spec for normal pool water (which assumes magnesium ions are much less concentrated than calcium ions), that the test as directed can not tell you what you need to know. Simply put, you would have to take the water sample into a lab environment and do a lot of calibrations and sensitive pH measurements to know when you've precipitated enough magnesium.
Taylor does sell a general hardness (GH) test that is much simpler than the CH test, but I'm not sure knowing MH + CH is all that useful. As a general opinion, I see no reason why anyone should use salt that has both magnesium and sodium chloride in it. There's not health benefit to do it and, as you can see, it makes a critical component of your pool water testing impossible to do. The good news is that your fill water has very low natural calcium in it so your pool is probably safe from any kind of calcium scaling issues but that is more the exception than the rule. If one lived in a high CH water area, not being able to clearly know your pool's CH value would be a real detriment.