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Mysterious indeed.

I saw that on another occasion, but thought it was "bug" in the DAZ Generation 4 figures or a problem DAZ vs Poser somehow. If you take a fresh M4, apply Morphs++,
set the Feetsize Morphform to -0.25, Create Full Body Morph, Dial that to 1.0, Memorize Figure, Restore Figure, the Feet shrink to -0.5.

Something is "out of sync".

But still, I think a figure should be able to be back-uped and restored properly. Especially if nothing else happens between backup and restore :)