Chak calls her Kathryn in The Eternal Tide after she is returned to him (clothed) from the dead and takes him back into her bed.
So. Her flesh and blood lover calls her Kathryn. I think Katie is only used as a family diminutive.
If you're intimate with someone, would you really still call them by a formal name?
Has the writer actually had an ongoing sexual relationship?
Sure, if that's the name they prefer. Not everyone is comfortable with their nicknames or diminutives.
Though I like this idea, perhaps it means they weren't really lovers?