I have not seen this issue nor have a solution for it. At this point I would recommend calling your region's Xerox Support Department to speak with a product specialist on the issue.
I have also started to encounter this issue recently on a couple of our C605s. Typically when we're trasmitted a fax header it prints it over the original image or reduces the image size to accomindate it. The test machine I'm troubleshooting on is no longer doing this regardless of embedded fax settings.
Have troubleshot fax settings for this issue for the last four hours with no resolution. If I can't figure it out here in another hour or two I'm going to factory reset the printer to see if it is something that came over in the last update. If someone from Xerox could advise that'd be great.
All incoming faxes are printing the last line on a 2nd page when there is adequate space on 1st.