Thanks! I changed the font on my Master Pages and it seemed to do the trick. What's odd is that particular font is used within other parts of the book and it displays fine.
That one looks as though the ‘fi’ ligature is missing from the PDF file. Either turning off subsetting on export to PDF or disabling ligatures in the Affinity file should solve the problem.
If you can share a single spread of the Publisher file (you could delete the body of the pages) with a working left side and a broken right side, one of us could figure this out for you.
I suggest comparing the left and right headers in Publisher. There has to be some difference between the two, perhaps the right side header has a version of the font that is no longer installed. It would look okay in Publisher but might show up as !Font name. Also check the Font Manager for missing fonts.
If all else fails, just copy the contents of the left header to the right and then edit the text as required.
Cheers