Windows 10, GeForce GTX 950M
Broken: 2.78b, 12b62b5
Short description of error
See the files:
stretch.blend contains a rig with a bendy bone "A", its custom handle references "tangent_A1" and "tangent_A2" (used to control and animate it), and a bone "Bone" with Copy Location constraint set to the middle of the bendy bone.
linked.blend contains the same rig linked as proxy. Both tangent bones are animated.
"Bone", instead of sticking to the middle of "A", jiggles around like crazy (no proper update) when scrubbing through the timeline.
The problem appears only when the bone layers are protected.
When I append the action back to the original rig file, everything works fine.
Exact steps for others to reproduce the error
Download both files into the same folder, open link.blend and scrub through the timeline.