"An illegal choice has been detected" when revising node

I have content of type X containing multiple paragraph bundles.
After the content was created, I revised the paragraph type, inserting an Accordion Group and an Accordion Item, pulling most of the existing fields into the single item.

When I edit the pre-field-group content, I receive the error "An illegal choice has been detected" upon saving. This happens whether I actually make any changes to the content or not. Prior to attempting to save, the Accordion group is collapsed in the form. After the error, the Accordion group is not collapsed.

Any content created AFTER the field group was inserted into the content type is fully editable; no error is thrown.

Why would this happen? Is there anything I can do to fix this problem?

Thanks!