Make sure ERG mode is turned on in Zwift. There is a check box for that when you start the training. In ERG mode the trainer has a little delay to adjust itself if you change cadence so you can create power spikes for short durations, but it should not ne sustainable. After you increase cadence power raise and in a matter of a second or so the trainer will decrease the resistance accordingly and power will get back to target. If you do not see this behaviour ERG mode is not on. You can also turn ERG mode on and off with the Zwift companion app, you will see the switch when you run a workout.