Maybe there is an issue with the high end of the nitrate test.
Differences in test colors can be tough to recognize. When colors are dark, seeing the difference between the color for 48 ppm (after the 80% water change) and 60 ppm (before) might be tough. Here is a way to test whether this is true in your case.
The next time you do a water change, measure the nitrate level of the new salt water to confirm it is very low or zero. If it is, mix 8 mL of tank water with 2 mL of new salt water to obtain a 20% dilution of the tank water. Measure the nitrate level of this diluted tank water sample. Measure the nitrate level of an undiluted sample of tank water. Perform these tests within minutes of each other. Compare the colors of the two tests. Are they very similar in color or easily distinguished? Doing this will help determine whether there is an analytical chemistry issue or a chemisrty mystery.
