What happens if it is in the type certification data but not in the POH I have no idea
I'm not the expert on this, but my gut feeling is that this situation is not supposed to exist.
As part of the certification process I think the certifying authority is required to take a look at the POH, specifically the "limitations" chapter, to verify that it complies with the regulations. And the minimum equipment list (whatever it is called) is part of that "limitations" chapter.
Of course for older/vintage aircraft, and non-certified aircraft this might not apply.