We've also been running into this behavior; html tags and classes get stripped out in CKEditor 5. I can see where normalization could be the culprit, but in our case, we have a clunky work-around when the behavior occurs. If resaving doesn't work after several attempts, we copy the content (Source), paste into a text editor, add the new class or html there, copy the updated content back. That usually works.
The fact that we can eventually save it indicates that normalization would not be the root cause. Let me add, this behavior occurs in nodes and webforms, plus we use Site Studio where it occurs in our components.
This started happening when we upgraded to CKEditor 5. We're currently on Drupal 10.2.4, but will be going up to 10.3 very soon.