Windows 7 x64
NVIDIA GeForce GTX 580
Broken: 2.79 5bd8ac9
Short description of error
Action Constraint doesn't evaluate B-Bone properties
Exact steps for others to reproduce the error
Create an Armature, and then an Action with keyframes on a bone's B-Bone properties channels. Assign an Action Constraint to said bone using this Action. When the Target Object is transformed, the constrained bone's Location/Rotation/Scale will change as specified in the Action, but any B-Bone properties animated in the Action stay the same.
In the attached .blend, the Action has keyframes for both Location and B-Bone Curve In Y. The Action Constraint on Bone.001 uses Bone as a Target Object. Move Bone +Y and Bone.001's Location changes as expected, but the B-Bone Curve In Y stays the same.