why am I getting regular upticks in stroke rate during my rowing activities while using a fenix 7s and a garmin HR chest strap

I get regular upticks in stroke rate while using my watch during rowing activities.  It has started about 2 months ago and I cannot figure out why it is doing it.  the upticks are regular and the same magnitude.  They are not natural and not associated with my rowing during the activitiy - they are regular and significant jumps in stroke rate and i and my crewmates would notice if this was actually happening.  I recently upgraded to fenix 7s from forerunner 735 and it is still doing it.  What could be wrong?  I am not sure how to upload a picture of an activity graph here.