There is a "bug" in the EF design as a whole: Entity Framework version 4.6.1x zzz-8 zzz.
Therefore, EF does not and will not like it even though we (and the RA models) know that it is a 1-1 relationship due to a Potential Key Restraint. Bad luck.
Among the "solutions" are:
modifying the model to a form that EF can understand (EF understands Code First, not RA). Granted, this might point to a "problem" with the chosen RA model, but that is unrelated to the inquiry.
Live with the improperly created multiplicity rules and "use with caution"; the grunt work can be wrapped, but it must be added manually outside of the automatically formed model.
Others, I wonder?
Shameless pitch for unanswered queries that address the same essential lack-of-feature