This is now working according to how BoneFollower was designed.
BoneFollower is supposed to follow a bone, not mimick it. (ie, it couldn't ever take on the internal inherited matrix of a Spine Bone, so no skewed inherited scale, shear, and less so for flip.)
That said, you're right, you could just add the negative scale.
I suppose we could add that and make it an opt-in thing if it doesn't muddy BoneFollower's functionality.
What do you think we could call the bool? "flipWithSkeleton"?
This is very simple to do though. You could do it yourself if you're in a hurry.