When you run on the treadmill on an incline, you need to set the incline in the app to get the correct power readings as Stryd can currently not detect incline (outside it relies on elevation change to calculate the correct power level). To set the incline, follow the following steps:
Open the Stryd app on your iPhone. Select the Calendar on the bottom of the screen.
Click the + icon (top right). Select "Start Run" > "Indoor Run"
Click on the up and down arrow icon.
Set incline. The Stryd app allows an incline from -50% to +50%.