Mitt Romney may not be a natural born citizen if George Romney was born in Chihuahua, Mexico with natural born Mexican citizenship and/or U.S. citizenship, natural born or not. Natural born citizenship requires no natural born allegiance to a foreign sovereign in competition with any allgiances to the domestic sovereign.
If George Romney was a citizen at the time of Mitt Romney’s birth, ditto his mother, then Mitt Romney is an NBC regardless of the type of citizenship his father held (NBC, Statutory Citizen by birth or or naturalized).
I have never heard the claim made that George Romney was not a US citizen, and I doubt that any such claim is about to be made now.