Gist For https://blogs.bitesinbyte.com/posts/Cosmos_DB_Sdk_System_Text_Json_Issue/

CosmosClient.cs

JsonSerializerOptions jsonSerializerOptions = new JsonSerializerOptions()
{
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
};
CosmosSystemTextJsonSerializer cosmosSystemTextJsonSerializer = new CosmosSystemTextJsonSerializer(jsonSerializerOptions);
CosmosClientOptions cosmosClientOptions = new CosmosClientOptions()
{
    ApplicationName = "Test",
    Serializer = cosmosSystemTextJsonSerializer
};
var client = new CosmosClient(endpoint, authKey, cosmosClientOptions);