
As you can see on the panel, Acrobat says that a field, db_sig_date (the signature date?), has been filled in after signing. This would mean that indeed the document has been changed after signing.
Could that actually be the case for your document?
If yes, then the signing party should change their work flow and first enter the date and sign thereafter.
If no, though, I.e. you still are sure nobody has changed that field after signing, there may be other causes. E.g. there might be some JavaScript code in the document that automatically fills the current date into that field upon opening the document. To find the actual cause, we would need the document in question for analysis. If you cannot share the original document, please ask the signer to create a similar document with dummy data.