We were unable to reproduce this issue based on the information provided. Went to single-player, started a new save on Siptah, picked Ymir, and got Ymir.
Then tried going back and starting a new save on Exiled Lands, picked Yog, got Yog.
We’ll need more information about this. @Verial, Would you happen to be able to take a video of you creating a character and showing you ending up with Mitra despite choosing something else during character creation?