I'd first check to make sure someone didn't mix and match the wrong base and swivel parts, or installed them incorrectly, before modifying the seats themselves.
Seats should swivel about a vertical axis so that seat angle remains the same whether facing forward or back -- or to the sides for that matter.
Many seats have a slide at an angle that raise the seat as it moves forward (for shorter driver/passengers), and lowers the seat as it moves back for taller people. If for some reason a swivel base was mounted at an angle (above the slide), I can see where the seat would change angle as it rotates. And seats shouldn't do that.
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