Sounds like there may be a conflict or error with saving the PDF form. . From the plugin’s EXPORT tab, are you able to EXPORT a PDF with the correct data merged to the PDF form? . Have the flatten and email settings worked on your installation in the past, and just stopped for one field map?
Depending on your answers, we may need to take a look at your installation, but first please reply here with the above info and screenshots of; . The EXPORT tab with the Form and Field Map selected and showing all the settings and at least the first mapped field. . The Under the Hood tab.
Or, provide login details on this form and we’ll take a look and fix it faster, if possible.