There were a few bugs in RC 2,6 that that could be causing this. They're fixed in the 2.6.2 beta. It might be worth a try to see if it fixes the problem issue (I opened your file with this version and it appears to be correct). Specifically it includes these fixes.
- Fixed incompleted segments with Bevel and Extend mode.
- Fixed bug with Bevel and corner angles that are exactly 180 degrees.
You can read a bit more about this version here