We've found that users who have Garmin watches and head units and use either the Stages power meter, or Garmin Vector power meter pedals, Garmin has given them a priority for cadence.
For those people, pedaling will begin showing real cadence over SmO2. Unfortunately, there's no work-around for this, currently. The cadence is coming through the power channel, so there's no way to stop it, and Gamin doesn't have any customizing, or priority settings a user can adjust. Other power meters and watches head units don't seem to have this problem.