You might notice that your power fluctuates while running on a treadmill, even though pace is very constant. The spikes in power are caused by changes in air pressure while you are running on the treadmill. Pressure changes are interpreted by Stryd as changes in elevation, causing power to fluctuate.
You can avoid these fluctuations if you record your run with the Stryd app your phone in the indoor mode or the indoor mode on either the Stryd Apple Watch app or the Garmin Stryd Workout App. Stryd will ignore the elevation data in these scenarios as it now knows that your running on a treadmill. Therefore, power flattens out.