.Net Core 3.1 json serialization Issue

Recently I’ve upgraded my .net core 2.2 projects to 3.1, dotnet core 3.1 using their own JSON serializer and

because of that my project was broken. But somehow I’ve managed to add some custom JSON converters and fix most of the issues. but the below one still remains.

when using .net core 2.2 my API response comes as below. This is the correct and exact response format I want.

{
"successString": "",
"errorString": "",
"lockedMessage": "",
"statusCode": "1",
"message": "Success",
"result": null,
"data": "0006429790"
}

but in .net core 3.1 it’s responded as below. It’s ignoring most of the properties. what could be the problem here?

{
"result": null,
"data": "0006429815"
}

NOTE: with newtonsoft, this works fine but I need to use default serialization here

Thank you.