I regularly have heart rate monitors disconnect from SYSTM when restarting a workout or finishing a workout and starting a new one and not be able to reconnect. This seems to be a common occurrence with other users as well, and various arcane nostrums involving cycling power, cycling Bluetooth, etc. only randomly seem to work about 20-30% of the time.
Similar to Saddlesaur, this hasn’t ever happened to me in any other cycling apps, even (somewhat ironically) RGT.
I suspect the SYSTM code handling Bluetooth connections is just some combination of kludgy, outdated, and/or not very robust.