1. A small XML sample with data that causes these linebreaks
2. A small XSLT transformation file that you use to create XSL-FO of
from the XML
3. The current resulting XSL-FO
4. The versions of your XSLT and FOP processor

If you are certain that your XML/XSLT transformation process is fine but
are unsure of the XSL-FO part, create a minified example XSL-FO (nr 3)
that illustrates your point so that we can have a closer look at it.
Also: do other XSL-FO processors produce the same output?

Alternatively, you may try the FOP list, if the issue turns out to be
FOP specific.

Kind regards,
Abel Braaksma

I have a really strange behaviour of FOP here:

Have a look at this page, sometimes it works, sometimes not:
I am using automated hyphenation.

http://spunkmeyer.de/linebreaks_page.pdf

Have you discovered something similar? How did you fix it?