Finally:
If you create a "New Group", you basically split a bodypart into sub-parts with new materials.
You can re-assign the original material with the grouping tool to the new group so that MAT files will work again, but it's safer to just save a copy of your re-grouped character to your figure library for future use, and export your new morphs as objects and use them with your original (un-regrouped) copy of that figure.
(Or use MorphManager to transfer your new morphs from the regrouped figure to your original one)