Hah, yeah. What @Eric said. My first ride or two I had all kinds of weird stuff happening until I realized I had my phone, my Garmin Edge 520, and Zwift all fighting over control of the trainer. So now the Garmin has the Vortex “sensor” disabled unless I’m using it specifically. I always start the Tacx app, connect to the trainer, calibrate, disconnect from the trainer, then start Zwift on the PC.
One last thing: make sure your rear tire pressure is consistent as that makes it easier to get a consistent calibration.
Based on my comparisons with both Power2Max power meter on my road bike and Quarq on my TT bike, the Vortex power readings are pretty spot-on as long as the calibration bar is very close to dead center in the app.