I did bump into this page. So in case others do, I overcame this very simply by running my-site-url/core/rebuild.php (you will need to enable it temporarily in settings.php).
Also happens when trying to 'add another item' to a Selection list field (in my case 'List (text)') and when trying to change the allowed number of values.
I have a theme based on the D10 starterkit and it does not have the full responsive menu layout out, including the hamburger symbol for small screens. The lack of this element is the one item that is stopping me moving my sites from D9 to D10. The module responsive_menus is very nearly there (it has a patch for D10 compatibility that works), so I am hopeful that I will not have to hang on too long.
Used patch #3 on a site migrated to D10. Everything behaving as expected.
Thanks for the patch. Have tried it on a site migrated to D10 and it behaves as I expected.
I cured the problem: any role using Gutenberg needs to have the "View the administration theme" permission enabled. But I do not understand why this has to be.
I am having the same problem with a Drupal 9 implementation.
If I login as administrator then everything works well. If I log in as another role with permission to use Gutenberg, the hmtl around the save button is completely missing. Comparing the rendered hmtl, I see that a complete div class="gutenberg-header-settings" which contains the save button is missing from the non-administrator role.
I have not had the time to investigate further,