This issue →
on Drupal core has a patch that changes the type of the JavaScript variable Drupal.ckeditor5.saveCallback from a function to a Map object. This requires changes on entity_embed code that uses it, otherwise the following error is shown:
An error occurred during the execution of the Ajax response: TypeError: Drupal.ckeditor5.saveCallback.has is not a function
This patch changes the how the Javascript code treats the Drupal.ckeditor5.saveCallback.
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This worked for me.
Without the patch, I was unable to edit a the caption after adding an entity embed.
However this patch conflicts with https://www.drupal.org/project/entity_embed/issues/3410132 🐛 Entity embed dialog can no longer use custom data- attributes in CKEditor 5 RTBC .