I don't think it is your filter as someone suggested. Filters don't filter out nitrate anyways, they create nitrate by converting ammonia to nitrite then to nitrate. So if anything, it shows you filter is doing it's job.
I think it may be nitrates in your source water. Do a full water test (ammonia, nitrite, nitrate) on your source water that you use for that tank. Odds are it may be going in already high, then your filter and beneficial bacteria are adding to that number to make it go even higher.
Another thought, maybe there is something hidden that is dead/decaying that you are not seeing, like some dead shrimp hidden in foliage of the plants. Maybe something under the substrate that is leaching out into the water like dead plants roots.
Last suggestion would be to have your water tested at LFS to see what their tests say. Maybe you have a bad test kit for whatever reason. I know you said it wasn't expired. It could have been contaminated or compromised some other way, who knows. Good to have an outside source check to see if they get same results. Maybe bring a sample of your source water and one of your tank water. Petsmart and Petco along with other places will test your water for free.