Over here Sketch does not seem to be respecting the Planar option. If I start sketch, have Planar on, OSnap the start of my Sketch curve to something off of the CPlane Z, then turn continue to draw (with or without turning OSnaps off), the rest of the sketch curve will be at CPlane 0 instead of the first OSnap pick. What I would expect is the same behavior as other curve tools where if Planar is on, the rest of the curve stays at the same Z value unless snapped to something else.