I too am skeptical that TNG would take on Muldaur from the beginning if she was only willing to do one season. I read somewhere that Diana left after S2 because she hated doing the show due to the cast & crew essentially giving her the cold shoulder (because they were unhappy about Gates’ departure). I hope that isn’t true, but given how hard the cast lobbied for Gates’ return I suspect it may well be.Apparently she didn't want to be in the opening credits, why, I don't know...
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Interwebz says: Diana didn’t want to make the long term commitment required to be a regular cast member - she never intended to be on the show for more than one year.
^^^ I'm not sure how true that is though.