The setter of JsonSerializerSettings.DateFormatString has a hidden side effect: It sets a field telling the writer (through the serializer) whether the property has been set - even if setting the property to null:

https://github.com/JamesNK/Newtonsoft.Json/blob/666d9760719e5ec5b2a50046f7dbd6a1267c01c6/Src/Newtonsoft.Json/JsonSerializerSettings.cs#L321

So when using AddNewtonsoftJson and setting DateFormatHandling = DateFormatHandling.MicrosoftDateFormat it will not be used by the serializer, because ShallowCopy sets DateFormatString (which takes precedence over DateFormatHandling) and thus triggers the above behavior: