Seems like i forgot a parameter that is required - yet it works without it for Long-positions, but not Short-positions.

                self.session.set_trading_stop(
                    category="linear",
                    positionIdx=2,
                    tpslMode="Full",
                    symbol=self.symbol,
                    stopLoss=str(int(new_stop_loss))
                )

Added the tpslMode="Full", and the stop is updated.