I'm in the process of testing various FO Processors. I created some test files, one of which was a single page invoice and a multi-page invoice. The first page has a full header and remaining pages have a small headers (logo only). The last page has a big footer with totals. If the invoice only has a few lines then the big footer needs to go on page 1 (so first page header and last page footer). To accomplish this I use a conditional page-master with the page-position="only" trait.
It works fine for multi-page invoices in Antenna House and XEP, but in FOP, I get the "only" layout for every page. This was supposed to be fixed in FOP 0.92, but still doesn’t seem to work. Below is a snippet from my XSL (full files attached)