A possible bug in QGIS 2.12

Working with my maps of a segmented DEM showing average slope direction and magnitude, and curvature with arrows, I noticed it broke once I upgraded to QGIS 2.12.
Unless there has been some change in the syntax of expressions that I didn't get from the QGIS 2.12 changelog I suspect a bug.

I made a minimal example, which I shared via GIS Stack Exchange and QGIS issue tracker.

QGIS 2.10.1. The width of the lines are proportional to the magnitude of the slope, and the colours are proportional to the 'longitudinal curvature' with blue for concave slopes and red for convex. No prizes for guessing the location since it's labelled on the maps at the links above.

QGIS 2.12. Styling is lost for the slope magnitude and longitudinal curvature, but slope direction is still shown.