Hi @Biju Mathew Thank you for posting your question.
This is a common question addressed the same way as for the SQL Server. SQL Server does not automatically rebalance data across files immediately after a new file has been added to the filegroup. Once you add a new file, it is empty at first, and lots of data writes needs to happen in time for SQL Server to naturally rebalance this in time. This article addresses this question well, there are many more articles in SQL Server literature that you can look up.
Hope this helps.
Regards,
Oury