Using a portal in a report at all could be a significant flaw. The limitations inherent to portals can cause problems on reports.
That said, the portal should work, but it will function from the context of the last record in your found set and that might not work for you.
In a trailing sub-summary, your context is the last record in that group, which means the portal filter is evaluated based on data from that record. (Assuming you are using data from that table in the portal filter at all.)
Maybe describe your scenario using the real-world application?
It depends on what the portal is realted to. I'm with phil and erolst, more information?