It seems there is a bug in WAS that was introduced with 18.104.22.168. I found this forum post on the subject.
Apparently, WAS looks for a resource in the WAR that matches the URL before allowing the servlet filters a chance to process the URL. Despite the fact that the design of the Servlet Filter mechanism is to allow web applications more control over their URLs and how they are accessed.
Some of the wording in these bug reports and the reply from IBM posted on that Springframework thread appear to indicate that IBM sees nothing wrong with this behavior. I disagree. The servlet specification does not place any restrictions on how a Servlet filter can transform a request URL so it stands to reason that no check for the existence of the referenced resource should be performed until the filter are processed.