http://map.project-osrm.org/1r9 The obvious route is to follow I-80 all the way. But instead it gets off and follows surface roads across Indiana.
Could it be avoiding barrier=toll_booth? The exit where it gets off (SR 66) is missing this. But why would it avoid toll roads without the user telling it to do so, and why wouldn't it be using the toll=* tag on the ways?