Looking at my values closer, looks like you are probably right.  My input image is 0 to 1 linear, so I wasn't using a shaper.  However, when I get to LAB space, there are negative values, which would explain why its all messed up going back to rgb right afterwards.

But what shaper would even make sense here?  My head hurts trying to think about the a*b* components going through a log curve or something. That doesn't really make sense, does it?