I’m not sure why you would expect a positive influence on FTP from changing diet. A higher-carb diet would probably give you more energy for short power efforts, but I don’t think you’d notice it in regards to FTP tests or endurance rides. You would more likely see an influence on your weight.
Besides using two different tests to measure FTP (whichbis likely the main problem), how did your training volume change? How many hours did you spend zwift racing, vs how many hours are you doing now? How did your TSS/CTL change? If you had a significant drop in volume, that could also explain a drop in FTP.