When I was in college nitrogen testing was rather tedious and complicated. I rather doubt that anyone has a spot test or instrument that is both affordable and accurate. But, who am I to ...?

I had the same test result and I too tried error checking the result by adding a few drops of a nitrogen compound. I used ammonium hydroxide aka common household ammonia. No change. This means either (1) the test is defective or (2) the test is an irreversible reaction that is finished by the time the test is read. Thus, adding nitrogen after the fact shows nothing. To check this test, add the nitrogen compound to the sample before adding the test reagent.

Adding ammonia before the test reagent made no difference. There is a precipitate, but it does not have the tell-tale color. So the question is what nitrogen compounds does this test detect?

Adding ammonia might change the pH to a range where the nitrogen test does not work. Some aquarium dipsticks test for ammonia. Others have nitrate, nitrite and pH on the same dipstick along with some others.

Since your are on the experiment forum let me share some of what we did in college agricultural and horticultural laboratories (This is not the whole soil test. This is for the soil solution, which is quite important but not discussed much on gardening forums.) Put some soil in some water and shake it up and let it settle of some significant period. Then filter, you can use a coffee filter, and test the supernatant.