Might be the way Bluetooth is implemented on your phone as far as the sensors reestablishing a connection when you are back in range and using the phone app.
My motoG7 was a pain trying to keep it's Bluetooth connection to my coffee cup. Sometimes it'd reestablish the connection and other times it wouldn't and I'd have to go do various things to get the connection. However since I got a Samsung phone, it's been no issue at all.