Changes made to document after Digital Signature while using Adobe Sign

A pdf form was made using Adobe Acrobat and a digital signature was applied that makes all fields READ ONLY. The document was then sent for signature using Adobe Sign. The signer was able to modify the text of the existing fields, then apply the signature without invalidating the digital signature.

I have attempted to modify the document in Adobe Sign before sending for signature and get an error that prevents the document from being sent because the digital signature is no longer valid. This did not appear to happen when the signer modified the document. Has anyone else noticed this or know a fix?