The problem is caused by incorrect order of retrieving a request attribute within a Portlet dispatched servlet.
First the PortletRequest (cached) attributes needs to be evaluated before delegating to the web container.
This is needed as the web container itself might have already set the attribute value before which only is stored in the PortletRequest attribute cache (within the PortletWindow).
In particular this failed when using a ResourceRequest which is from the portal forwarded to the portlet.
If that portlet subsequently dispatched again using an include, Websphere still saw the initial dispatcher type (forward) instead of the current type (include), causing it to fail finding the appropriate filter.
Fixing this required changes to both the pluto-container (see:
PLUTO-598) as well as the jetspeed-portal artifacts.