Data passed to the Sanitization function is empty

The seemingly related question offered not much help except for the first answer saying it could be Nonce related, but it doesn’t help in finding out how…? (neither do I understand the documentation).

The issue started after I changed my settings page to a Tabs-based page.

Although I get the “Settings saved.” alert when the page is posted but nothing gets written to the DB.

When I use the following bit of code to check for the $input that is passed to the sanitize_callback function:-

NULL is printed in the error_logs.

Thank you!