Hi @Anonymous ,
Based on the issue that you mentioned, I agree with @SeanHenderson 's thought almost. Although the Items have been removed from your CDS Entities, but previous selection history has been cache within the LookUp field in your Model-Driven Entity form.
As an fixed solution, please consider edit your Entity form, remove the LookUp field, save and publish your Entity form. And then, edit your Entity form, add the LookUp field, save and re-publish your Entity form. After that, open your Model-Driven app again, check if the issue is solved.
Note: Currently, there is no direct way to clear the previous selection history of a LookUp field.
Best regards,