The checkout, when assembling the payload for the tax service, uses the document data (whether corporate or not) in place of the email. In this way, the clientEmail variable retains the definition of the document that was received and not the email itself.

Make a simulation for the FFM by correctly passing the customer data in the clientProfileData and notice using the debug mode the request that is made to the tax service the clientEmail variable using the information that was passed in the corporateDocument or document (depending on the type of document)

Don't take into account the clientEmail, but the email that is inside the clientData. There the data is correct.