All, let me add to the conversation.........I forgot about a topic years back where someone correlated this in the lower HR ranges to interference with the cadence signal. Can happen with a gps or even with a computer/Zwift setup.
When the HR and cadence are within a certain range of each other, for some reason the bluetooth and/or ANT signals get confused with each other. And the HR being shown in the dips is possibly it choosing your cadence at that moment instead.
I've actually seen this live happen in warmups for Zwift workouts where my cadence is in the 90's and my HR is in the 90's to around 110 or so. I'll suddenly see my cadence jump to like 120 or 130, then fall back. Then the HR fall to 90 or so and go back to the 120/130 I know it is.
This can happen on any device with the HR/cadence thing, so changing devices might not do it. Only thing you COULD do is log a separate workout on a smartwatch using HR only then compare the file to the workout file that has both cadence and HR.
It's called "cadence lock". A few replies to this DC Rainmaker article mention it:
https://www.dcrainmaker.com/2010/04/...eart-rate.html
Apparently for running it can happen also, spiking though since run cadence is 2x bike cadence since it counts each stride............so for running your HR may go from a Z2 of 140 up to 160/170 as it locks onto your run cadence..........of about 170.
Running writings: Cadence lock: Why GPS watches have a hard time measuring heart rate during running
Personally I've seen both happen. In both cases, intentionally altering my cadence a large amount causes it to unlock from my cadence on the HR. So running I will take long slow strides and on the bike slow to a grinding out of saddle climb of like 45 rpm. Then it usually re-grabs the HR properly.