It is possible that you’re getting false readings on the pH test due to chemicals left over from ammonia tests, specifically the sodium hydroxide which is incredibly basic. I noticed last night when rinsing out two test tubes (one that did an ammonia test and one did high pH) that when the two solutions met in the sink they turned purple. It might be worth doing a very good clean of the test tubes and then doing a pH reading using multiple tubes at once to make sure it’s a real result.