Hyperlinks between RH projects linked with FM books

HI, I want to link multiple framemaker books to robohelp and use each book as a child project in a master project. However, there are multiple hyperlinks between the framemaker books. I have created individual RH projects, and linked a single book to each project.  I generated them all into a master project (as usual), but the hyperlinks between the projects (that worked fine between the PDFs generated from the original Framemaker books) are broken in the output. I check the html of the hyperlinks in RH, and while the text is recognized as a hyperlink, the href tag is missing.

Note: All links with target and destination in the same book / project work fine.

I could create a single book containing all these books, and then link this book to FM, and this would probably avoid the issue, but it wouldn't be a good solution as I would then have  workflow / source control issues.  Also, I would obviously prefer not to have to manually repair the links in RH, due to time contstraints.

Any suggestions?