Hi,
In our testing with v15.5 and WordPress v5.6, we were able to enter a new slug and save it and the field still showed the new slug. It was not blank.
To clarify, the issue still happens if you use a different browser and you do not have any plugins which impact the permalink?
If so, That sounds like a plugin or theme conflict. Can you try and gather as much information for us as possible? Please perform the following:
1. Make sure you are using the most recent WordPress Core, v5.3.2. If you are not sure what version you have or how to update, your host provider can help. Also, be sure to update your Yoast to v12.2 and update your non-Yoast plugins.
2. Make sure the issue doesn’t persist when Yoast SEO is disabled (disabling Yoast will not impact your site; just be sure to re-enable it when you are done). Can you change the slug using the default WordPress method? While not our guide this one does show a way to change the slug: https://wpengine.com/resources/wordpress-permalinks/#2_Change_the_Slug_for_an_Individual_Post_or_Page.
3. If the issue only happens when Yoast is enabled, please Check for conflicts. Install the Healthcheck plugin and enable troubleshooting. Then switch to a standard theme like 2020. Does the issue resolve? NOTE: This will not affect how users see your site.
4. Check for JavaScript errors with your console.
If you find any JavaScript errors related to Yoast SEO or if there is a conflict with a plugin or a theme, you can create a new GitHub issue?for our developers. Please report the issue to a third party developer as well.
If you didn’t find any conflicts or errors, we think the issue is specific to your site. We’d need to investigate further but are unable to do so on these forums. You can purchase Yoast SEO Premium?and receive our Premium email support and we can help you further.