I hope they fill fix this issue soon. A temporary solution may be not to use any sub-category in the type of activity. For example if cycling don't use, indoor or road bike, just cycling. If running select only running and not tread mill, trail etc. More here:
https://forums.garmin.com/showthread.php?74807-Cadans-graph-empty
https://forums.garmin.com/showthread.php?72943-Gamin-Connect-Not-Showing-Cadence-Chart