InvalidToDefaultEnumConverter

EnumExtensions.cs

public static class EnumExtensions
{
    public static T? GetDefaultValue<T>() where T : Enum
    {
        var enumType = typeof(T);
        var defaultValueAttribute = Attribute
            .GetCustomAttribute(enumType, typeof(DefaultValueAttribute)) as DefaultValueAttribute;

        if (defaultValueAttribute is not null &&
            defaultValueAttribute.Value is T)
        {
            return (T)defaultValueAttribute.Value;
        }

        return default;
    }
}

InvalidToDefaultEnumConverter.cs

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

public class InvalidToDefaultEnumConverter<T> : JsonConverter<T> where T : struct, Enum
{
    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        object valueSelected = reader.TokenType == JsonTokenType.Number 
            ? reader.GetInt32() : reader.GetString();
            
        if (Enum.TryParse(typeToConvert, valueSelected.ToString(), true, out object? result))
        {
            if(result is null)
                return EnumExtensions.GetDefaultValue<T>();
                
            return (T)result;
        }
        
        
        return EnumExtensions.GetDefaultValue<T>();
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value.ToString());
    }
}

Program.cs

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.Converters.Add(new InvalidToDefaultEnumConverter<EnumT>());
    options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
});