Garmin Instinct Solar Inconsistent Intensity Minutes

Second Garmin Instinct Solar.  First one did not record intensity minutes.  Second one does record SOME intensity minutes but it has a mind of its own.  Daily walks almost carbon copy from day to day, at least 3 each day of about an hour each..  HR very much showing the same daily pattern.  However, the watch recorded one day 68 minutes and the next few days nothing.  It makes absolutely no sense.  To make it even more puzzling, I have a Garmin Vivosport with identical HR settings, zones, etc. and it records intensity minutes without failure (under identical use).  Can anyone bring some light to this matter?  Maybe Chris at Garmin? I have plenty of information on Garmin Connect for them to take a look and see exactly these inconsistencies.