If you are using Stryd with Zwift, you may experience that Zwift continues to indicate a speed even when you stop moving. Below is an example of Zwift configured to use ANT+. The chart has data from Garmin, Zwift and Stryd (offline sync). You can see that Zwift continues to report speed even as Stryd and Garmin do not. This is a known problem with the Zwift app.