If you run with your Polar watch that supports running power (for example, Vantage V, Grit X, PacerPro) without Stryd, the watch will revert to its own running power algorithm. Vantage V power will always be higher than Stryd power.
The Stryd platform can currently not detect that power was not from Stryd when you import your data into the Stryd platform. As power is higher with the Polar, your critical power may be adjusted upward even though you are not getting fitter.
You can exclude runs from your Critical Power calculation so that your critical power and zones are correct again.