System.Text.Json.JsonSerializer ignores DictionaryKeyPolicy when serializing dictionary keys with non-primitive values

This is a known bug in .NET Core 3.x that has been fixed in .NET 5. See:

• System.Text.Json: DictionaryKeyPolicy not applied to keys that contain non primitive types #30889

• Possible DictionaryKeyPolicy bug when serializing Dictionary<string, object> #43143.

• https://dotnetfiddle.net/ycRUPy for a demo fiddle of your code working correctly in .NET 5.

If you cannot move to .NET 5 you will need to introduce a custom a custom JsonConverter similar to this one from the Microsoft documentation. The following should do the job:

public class DictionaryWithNamingPolicyConverter : JsonConverterFactory
{
    // Adapted from DictionaryTKeyEnumTValueConverter
    // https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?pivots=dotnet-core-3-1#support-dictionary-with-non-string-key
    readonly JsonNamingPolicy namingPolicy;

    public DictionaryWithNamingPolicyConverter(JsonNamingPolicy namingPolicy)
        => this.namingPolicy = namingPolicy ?? throw new ArgumentNullException();
        
    public override bool CanConvert(Type typeToConvert)
    {
        // TODO: Tweak this method to include or exclude any dictionary types that you want.
        // Currently it converts any type implementing IDictionary<string, TValue> that has a public parameterless constructor.
        if (typeToConvert.IsPrimitive || typeToConvert == typeof(string))
            return false;
        var parameters = typeToConvert.GetDictionaryKeyValueType();
        return parameters != null && parameters[0] == typeof(string) && typeToConvert.GetConstructor(Type.EmptyTypes) != null;
    }
    
    public override JsonConverter CreateConverter(Type type, JsonSerializerOptions options)
    {
        var types = type.GetDictionaryKeyValueType();

        JsonConverter converter = (JsonConverter)Activator.CreateInstance(
            typeof(DictionaryWithNamingPolicyConverterInner<,>).MakeGenericType(new Type[] { type, types[1] }),
            BindingFlags.Instance | BindingFlags.Public,
            binder: null,
            args: new object[] { namingPolicy, options },
            culture: null);

        return converter;
    }       
    
    private class DictionaryWithNamingPolicyConverterInner<TDictionary, TValue> : JsonConverter<TDictionary> 
        where TDictionary : IDictionary<string, TValue>, new()
    {
        readonly JsonNamingPolicy namingPolicy;
        readonly JsonConverter<TValue> _valueConverter;
        readonly Type _valueType;
        
        public DictionaryWithNamingPolicyConverterInner(JsonNamingPolicy namingPolicy, JsonSerializerOptions options)
        {
            this.namingPolicy = namingPolicy ?? throw new ArgumentNullException();
            // For performance, cache the value converter and type.
            this._valueType = typeof(TValue);
            this._valueConverter = (_valueType == typeof(object) ? null : (JsonConverter<TValue>)options.GetConverter(typeof(TValue))); // Encountered a bug using the builtin ObjectConverter 
        }

        public override TDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.Null)
                return default(TDictionary); // Or throw an exception if you prefer.
            if (reader.TokenType != JsonTokenType.StartObject)
                throw new JsonException();
            var dictionary = new TDictionary();
            while (reader.Read())
            {
                if (reader.TokenType == JsonTokenType.EndObject)
                    return dictionary;
                if (reader.TokenType != JsonTokenType.PropertyName)
                    throw new JsonException();
                var key = reader.GetString();
                reader.Read();                  
                dictionary.Add(key, _valueConverter.ReadOrDeserialize<TValue>(ref reader, _valueType, options));
            }
            throw new JsonException();
        }
        
        public override void Write(Utf8JsonWriter writer, TDictionary dictionary, JsonSerializerOptions options)
        {
            writer.WriteStartObject();
            foreach (var pair in dictionary)
            {
                writer.WritePropertyName(namingPolicy.ConvertName(pair.Key));
                _valueConverter.WriteOrSerialize(writer, pair.Value, _valueType, options);
            }
            writer.WriteEndObject();
        }
    }
}

public static class JsonSerializerExtensions
{
    public static void WriteOrSerialize<T>(this JsonConverter<T> converter, Utf8JsonWriter writer, T value, Type type, JsonSerializerOptions options)
    {
        if (converter != null)
            converter.Write(writer, value, options);
        else
            JsonSerializer.Serialize(writer, value, type, options);
    }

    public static T ReadOrDeserialize<T>(this JsonConverter<T> converter, ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => converter != null ? converter.Read(ref reader, typeToConvert, options) : (T)JsonSerializer.Deserialize(ref reader, typeToConvert, options);
}

public static class TypeExtensions
{
    public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)
        => (type ?? throw new ArgumentNullException()).IsInterface ? new[] { type }.Concat(type.GetInterfaces()) : type.GetInterfaces();

    public static IEnumerable<Type []> GetDictionaryKeyValueTypes(this Type type)
        => type.GetInterfacesAndSelf().Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IDictionary<,>)).Select(t => t.GetGenericArguments());

    public static Type [] GetDictionaryKeyValueType(this Type type)
        => type.GetDictionaryKeyValueTypes().SingleOrDefaultIfMultiple();
}

public static class LinqExtensions
{
    // Copied from this answer https://stackoverflow.com/a/25319572
    // By https://stackoverflow.com/users/3542863/sean-rose
    // To https://stackoverflow.com/questions/3185067/singleordefault-throws-an-exception-on-more-than-one-element
    public static TSource SingleOrDefaultIfMultiple<TSource>(this IEnumerable<TSource> source)
    {
        var elements = source.Take(2).ToArray();
        return (elements.Length == 1) ? elements[0] : default(TSource);
    }
}

Then serialize as follows:

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DictionaryKeyPolicy = JsonNamingPolicy.CamelCase,
    Converters = { new DictionaryWithNamingPolicyConverter(JsonNamingPolicy.CamelCase) },
};
var serialized = JsonSerializer.Serialize(dictionary, options);

Demo fiddle here.

By default MVC uses camel-case in ASP.NET. However WebAPI is not configured to use camel-case. If you are on WebAPI can you try adding this to your Startup.cs

services.AddMvc();