IS-IS Level1 Router allways take the route towards the closest L2 Router, when trying to reach a destination outside the AS. That is the reason why the return-traffic takes a different way here.
The problem occurs because Level1 and Level2 topologies are examied separatly by the respective routers. The solution is to hand down Level2 routing-information about the destination networks into Layer1 routing, so it can be included into path selection.