Keywords not working

Good morning. I have a problem with keywords on my site. I have been using Rankmath SEO for a long time now and have never had any problems. After the last WordPress update I started having trouble with keywords. When I have finished creating a new article or a new page, I set the ‘Keyword’ for the article or page in the appropriate section of Rankmath Seo, I go to Edit Snippet, add the correct ‘description’, as suggested, I get a very high value, almost always close to or above 90 points. I then move to the ‘Publish’ section, click ‘Update’. The page or article is published, but at the end of the upload I get a very low SEO because in the appropriate SEO field the Keyword has disappeared into thin air. I try several times to repeat the operation but every attempt to see the stored keyword fails. I have already tried to uninstall Rankmath Seo. Yesterday I also upgraded to the Premium version again and from there I reinstalled the plugin updated to the latest version 1.0.54.1 by reactivating it, but the error persists. My theme is updated correctly, to the latest version available (update made yesterday morning). PHP is updated to the latest version available, 7.3 on Siteground. How can I go about solving this problem?