There is a case when Content Entity Type constraints do add error messages during form validation. Those are keyed with empty string in form_state_errors array in comparison to field level errors, which are keyed by parents path to field. There is a case when $error_key can be calculated as an empty string, thus doing a collision and grabbing top level form error and assigning as an error for each $untriggered_dependents fields. Later on this throws all the remove\add back logic under the bus and dismisses the top level form errors as result.
My addition to patch #149 add a simple check for non emptiness of $error_key, as top level form errors should not be matched against any field.
Patch re-roll for Drupal core 10.2
As of patch https://www.drupal.org/project/embed/issues/3310328 📌 When CKEditor5 is installed, only allow SVGs for button icons Fixed in Embed module all buttons require and SVG icon to be displayed.
I have amended the patch to use CKEditor5 icon.