Hello @mahantappa,
Thank you for contacting the support and sorry for the inconvenience caused.
Can you please confirm if you are using the Easy Table of Contents plugin? If yes, there was an issue with their previous version that was causing conflicts with other plugins. They have already released an update with the fix. Please update the plugin to fix the issue.
If you are not using the plugin then please check if you are using a Firewall, as it may prevent saving the changes.
Please whitelist the route given below in your firewall.
/wp-json/rankmath/v1/updateMeta
Whitelisting the route should fix the issue.
You can check this link for more information:
https://rankmath.com/kb/meta-data-not-saving/#firewall
Let us know how that goes. Looking forward to helping you.