JsonIgnore is being ignored

Question

I am using the GeoJsonSerializer as I need to serialize several geomtetries.

 JsonSerializerSettings sett = new JsonSerializerSettings { ReferenceLoopHandling = ReferenceLoopHandling.Ignore }; // ignore Self referencing loop
 var serializer = GeoJsonSerializer.Create(sett);
 serializer.Formatting = Newtonsoft.Json.Formatting.Indented; //pretty print
 using (StreamWriter file = File.CreateText(jsonFile))
 {
     serializer.Serialize(file, cities);
 }

and my class has this:

[JsonIgnore]
[NotMapped]
public string? OriginalSource { get; set; } = "UNK";

[XmlIgnore] // tempo
[JsonIgnore] // tempo
[DisplayFormat(NullDisplayText = "Null")]
public NetTopologySuite.Geometries.Point? Location { get; set; }

Yet these fields still end up in my JSON output. Is there another attribute that I am missing?