Hey @crashmartin, also Welcome to the Forum and to Sufferlandria. You can never leave. I had a single sided 4iiii Precision on my bike a while back and never had that issue. My facetious thought is that you must have been doing the single leg drills
Seriously though, have you checked to be sure you are using the latest firmware for the 4iiii and is the PM being picked up by the Garmin for both Power and Cadence and does it register the correct cadence while you are riding? If SUF is showing the right cadence but Garmin isn’t, I suspect it has something to do with how the Garmin is recording it, so, be sure you are using the most recent firmware for the Garmin as well as the 4iiii.
Last bit of useless advice…if you still can’t get it sorted, you may want to reach out to 4iiii support directly. I’ve found them to be super responsive and very helpful whenever I needed them.
Edit: I see you updated the watch firmware so that may also solve the cadence issue. I am curious how this turns out. Please come back and let us know