In four Frame files, the Solbook conversion process replaced line break directives with the word "linebreak" or "preflinebreak" (the name of the directive in Solbook). (There may originally have been more, but we must have caught them before.)
The files are jsf-facelets.fm, jaxrs.fm, persistence-basicexamples.fm, and security-javaee.fm. I just happened to notice one in the Facelets chapter and found the rest by searching for "linebreak" in the tutorial PDF.
I'm checking in fixed versions.