I had something similar happen to me in the past. Just warming up, soft pedaling at around a 60 cadence, it showed I was producing 700 watts! Later that ride, I earned the "1.2 Gigawatts" extra credit badge, and set a new KOM as well as grabbing a green sprint jersey. After a bit, the program caught on and stopped recording my efforts.
I was using a dumb trainer at the time, so I went into my settings and tried to re-calibrate it in the program with no luck. Chose a different trainer, then chose mine again. No change. I finally gave up and bought a smart trainer.
Later Zwift came out with a notice that their profile for the trainer I had been using had gone wonky. My neighbor is using it successfully now.