Can you please open the Developer tool >> Network tab and update the post? The Network tab will show the updateMeta call, if it returns a status other than 200 then that means there is some problem.
Hello, I am experiencing the same issue. I ran a network process as you mentioned. The only process that was above 200 was post.php. The status code was 302 with a yellow circle. All the processes that showed a 200 status were green.
Any help would be appreciated. I can’t save anything from Rank Math plugin on classic editor or Gutenberg.
P.S. In the meanwhile, assuming you are using the latest version of all the plugins, themes, and WordPress (please update if not yet), it would seem like a conflict with one of the plugins or the theme you are using.