I have wondered about this. I have never had a disconnect during strength training, HIIT, mindful cool down, or yoga sessions, but have at least one during every treadmill session. I speculated that there was some kind of interference from the treadmill itself. I moved the ATV and went from multiple disconnects to just one per 30-minute treatment workout. I'll see if there is another location that works better, although there's only so many ways to position the TV, ATV, and treadmill.
BTW, if I notice that a complete disconnect is imminent, with heart rate and calories no longer updating, I have been able to reconnect by pausing the workout from my watch and/or on the ATV, waiting a couple seconds, and resuming the workout. I say "and/or" as I haven't figured out if there is a specific combination and order of pausing/resuming on the watch and the TV. And I usually have to do this more than once. It's still frustrating, but at least I have been able to complete the workout rather than having it end prematurely.