I see what Waterdrop and yourself mean now..... very strange. Is that as high as you nitrate test kit measures? Try doubling the sample, add the drops and measure the KH, then half the value
I'd 90-100% waterchange, and monitor it over a few days... It could be that the KH was lower out of the tap when you started the cycle, and has since risen, making it apear that KH in the tank has dropped considerable....
All the best
Rabbut