There is a bug with the NLR system, given that it was made by EMP, which sometimes causes NLR to appear for admins, however a user won't see the NLR on their screen.

It seems like you did get NLR from dying in the district, however you did not get the banner on your screen telling you that you had NLR. If you did return to the district, you did in fact have NLR, so the warn was valid

If the timer was almost up and you did wait it out mostly, however, then I would tell you to appeal the warning, as it is difficult for you to know exactly how long the NLR would be, however I doubt this is the case, given that you ended up with a warning.