That will create a debug.log file in your /wp-content/ directory which you can examine in any text editor. Try editing the same page again and then inspect the log. If you don't find the debug.log file it means it didn't generate any warnings or errors.
Let us know what you find so we can see how to proceed.
I turned on debug mode. Also noticed that if I edit the Yoast SEO content on a custom post, that WILL save every time, but the edits to custom fields do not. They save sometimes. Maybe one field will save, but another will not. If you keep making the change, it might eventually save, but sometimes it takes 5+ attempts.