Ken, Unlike the Nike Frees which don't have a structured heel cup, Inov-8s do havc a structured heel cup ... it's an apples and oranges situation.
Structured heel cups which would make it very difficult to bend down the back of the shoe to go in to slipper mode. Is that how you split yours? or was it a quality control issue from just normal use?
Btw, I've logged miles in slipper mode on the Frees (relatively level terrain). YMMV