Interesting.  I can't reproduce it, but i haven't checked it out fresh yet.
I can however confirm that my copy of the repo (https://github.com/jbjonesjr/mapfish-print/blob/xyz_url_format/src/test/java/org/mapfish/print/map/readers/XYZLayerTest.java) is showing the same issue.  I'll figure out how to add a patch.