Are you sure your HR wasn't really 85 bpm?
I had a one time only 10 or so minutes of my HR dropping to the mid 80's when it should have been better than 130 bpm. During that time I could tell it was my heart as my vision was borderline on going blurry and I was ever so slightly feeling faint.
But I'd agree that when my Garmin soft straps are about to go, they do odd things for awhile till I get tired of messing with them and replace them. Usually last me over four years. My current is about to be six years old. It's had an odd moment or two recently.