Okay.
I do see the behavior you have. I actually had LV12 and LV13 open on my PC at the same time. When I tried replicating the VI, it was in LV2013, and the differences were explained by the dragging and dropping.
When I tried your VI, it opened up in LV12, and I see the behavior you describe. (lines grouped in the dialog, but the graph not showing the bus). I saved the VI and opened in LV13, and it worked just like it should.
I think you came across a bug in LV12.
It might be CAR#
| 378888 | The Mixed Signal Graph does not correctly label plots in LabVIEW 2012 |
http://www.ni.com/white-paper/14541/en/ says this was one of the bugs fixed in LV 2013.