With Zwift, I have to be VERY careful in what order I prepare myself for a ride. If I get dressed, then open the app, then fill my water bottles my HRM will report onscreen way under actual numbers (interestingly enough, it doesn’t show up wrong when exported to strava and trainingpeaks). If I power on my power meter cranks, then let them go to sleep, I have to re-enter the pairing screen to get them to work again. Entering the screen will cause them to come back (I don’t have to re-pair them, however, if I do not I will have a drop at some point in the ride)
The TL;DR of it is that if a device is connected during pairing, there is a bug that occurs if it gets disconnected then reconnected- for instance if the rider walks away and then comes back.