I noticed a glitch on my hr while doing a workout. It seems fine at the start of the activity but few mins after it goes flatline and I have to disconnect, turn off and on my hr sensor (my watch), and reconnect it again to TD app. It happened twice already but I haven’t tested using a different hr sensor.
I have seen HR sensors do this, I have a Garmin 955 and even it does this some times but I can say I have never seen it for that long a period of time. Since you are the only one that has reported this I would think it is more likely related to your watch but never can be sure.