In a perfect world, yes, it should be the same. But, due to limited numerical precision (32bits floats used in G’MIC), the result may differ a little bit from the original image. That’s true for almost all non-trivial color space conversions (I suppose RGB<->CMY is simple enough not to lose any precision, but that’s probably the only one).
If you use
-rgb2lch instead of
-rgb2lch8, you’ll probably get a slightly better precision though.
I’m not sure why ImageMagick returns 0, I guess this is maybe because it detects nothing is done between the two color conversions. Or maybe it is using double-valued (64bits) images instead of floats.
In any case, having a perfectly reversible image transformation is almost impossible when the transformation is complex enough (this is true for all theoretically inversible image processing operators, e.g. Fourier transforms, …).