If there is a lot of nutrients in the water this might happen. You might want to easy back a little on the nopox. Don’t reduce nutrients too fast.
Keep in mind the hobbyist grade test kit for nitrate can be quite inaccurate. It can be difficult to tell the difference between 5 and 10. I wouldn’t worry too much about it if it’s around 5ppm. You just need to figure out the maintenance dosage over time. Maybe measure once a week and adjust in small amounts. Also need to measure phosphate when carbon dosing. Either phosphate or nitrate depletion can be very damaging to corals.