I have a hunch about the stuck HR...one of the things i have noticed about my wahoo ticker is that when i descend there is so much air flow that i am pretty sure all the sweat on my chest is completely evaporated. pretty sure this is causing the strap electrodes to competely dry out. it lasts until i begin to slow down again and need to put in more effort to keep going, that is, i begin to sweat faster than it can be evaporated away. this is just my theory anyway based on the graphs i see when i get back from riding.
maybe the same for you when the reading seems to stick.