With the recent update to Rank Math, they've integrated it with Gutenburg. That it is moved to the sidebar. This in confusing, and seems annoying to many as well. So much that Rank Math published a filter to disable this layout.
To fix this, and get back the good old layout for the plugin settings. Simply add below code (If you're using Oxygen builder) add it to custom-function plugin. Or use code snippet plugin.
This is recommended over editing theme's Functions.php
How to Move Rank Math from Sidebar to Box Under Content
/** Disable Gutenberg Sidebar Integration **/ add_filter( 'rank_math/gutenberg/enabled', '__return_false' );
And there you have it. The good old Rank Math layout that we all loved.
If it ain't broken, don't fix it.