(*) The complete tittle would be: "Bone isn't affected by influence keyframe if the same action is shared with another armature with bones with different names".
- Open the file.
- Move in timeline and see what the bone (armature "B") does with its constraint.
- Select armature "C".
- Move across the timeline in DopeSheet: bone of armature "B" isn't affected by influence keyframe
The channel of the armature "C" gets red underlined. So, the Action is affected of this "red-underlineness"... so, armature "B" is affected too.
I know that this is a kinky bug... it is not normal/suggested to use the same action in different armatures with different bones. I discovered this bug by doing that, by mistake.
Maybe it is not usefull to put time on fixing it, because it is not a normal configuration. You guys decide.