Hello,
Thanks for contacting us and sorry for any inconvenience that might have been caused due to that.
I can check that the updateMeta route is returning a 403 error which means that it is getting blocked on your server:

Please get in touch with your host and ask them to whitelist the route: /wp-json/rankmath/v1/updateMeta?_locale=user.
Please check out more about this in our guide here: https://rankmath.com/kb/meta-data-not-saving/
Hope this helps. Please let us know if you have any other questions.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.