Thanks for the feedback.
Actually, it's a bug in the swimlane management. Unfortunately, it's a bug-by-design : meaning that the way we manage swimlanes cause this kind of line crossing. (So it's not a bug, it's a feature :-)
We are going to think about a way to solve this, but it's going to be long because we have to redesign many thing. I'm not even sure that we will be successful here. Sorry about that.
In your example, there is a second bug : the "end split" should happen in "Master" swimlane, not in "User" swimlane. This one has been fixed in last beta http://beta.plantuml.net/plantuml.jar