Hmm, I would certainly also have expected the rating, colour label, etc. to have been updated when you perform an "Update IPTC/XMP" of a RAW+JPG pair; for me those things are also part of the IPTC/XMP info. Could you perhaps make this an option?
Another thing in this respect: even though I have selected "When viewing photos in RAW+JPG mode use the metadata from the RAW file", the ratings and colour label are taken from the JPG... This combined with the fact that these aren't synchronised when you perform the IPTC/XMP of the pair, leads to unexpected results.
(Note: using 4.5.1.1)
By the way, so long as you are viewing RAW+JPG pairs, updates to the colour label and rating are synchronised, so for now this is a good work-around to keep them synchronised.