I had an issue this morning where my 4iiii PM was connected to my head unit and calibrated successfully, started my ride but the power and cadence didn't display and the icon was flashing to indicate that the PM wasn't connected. At the end of my ride I saved the activity and looked on Garmin Connect which showed that the PM and Cadence data must of been recorded within the FIT file.
Hopefully this was just a one off. Currently using 6.73 and this is the first time that this has happened.