Old ckeditor5_embedded_content plugins now don't work

Problem/Motivation

So following the steps on https://www.drupal.org/project/ckeditor5_embedded_content → we upgraded our module uswds_ckeditor_integration to use embedded_content but now none of the plugins appear to be working.

Warning: DOMDocumentFragment::appendXML(): Entity: line 8: parser error : error parsing attribute name in Drupal\embedded_content\Plugin\Filter\EmbeddedContent->process() (line 104 of modules/contrib/embedded_content/src/Plugin/Filter/EmbeddedContent.php).

Steps to reproduce

On latest D11 release
Install uswds_ckeditor_integration
Update any text format to use embedded content plugin
Enabled filter
Create a page
Notice nothing renders

Proposed resolution

N/A

Remaining tasks

User interface changes

API changes

Data model changes