The BRep is a Sweep1 surface as shown in the picture. The result of Brep|Plane is different between R6 and R5. The R5 one is correct, while the curve in R6 is incomplete. I believe the problem is associated with Rhino internal implementation and has nothing to do with GH.
PS: I just tested the script with the latest SR RC. The result doesn’t change.
In Rhino 6 (6.2.18065.11031 / 6.4.18093.10341):
In Rhino 5 (5.14.522.8390):
wrong.gh (68.1 KB) (i have internalized the geometry)