The reported issue was caused by a backward incompatibility due to careless
code changes made when per-material Freestyle line colors were introduced
in rB7915d7277ac8c605f016f30f943080556244fb59. In 2.71 line style
Material color/alpha/thickness modifiers was retrieving alpha transparency
from Material.alpha, whereas in 2.72 Material.line_color[3] was referenced.
The present fix reverts the aforementioned code changes.
This issue is a regression from 2.71, so the fix is appropriate for inclusion in
the 2.72a release.
f4a260fb3c