If you loose BT connection during track recording only when the display is OFF for at least 3 hours you may have a battery power saving setup issue. Following solutions do not help if you don't have stable BT connection during track recording with display allways ON.
If you still have issues with loosing BT connection during longer time in standby, you could try following steps (without warranty it'll help at all):
If everything above fails, you may need to use display always on as a workaround. You can reduce the screen time if you turn your display on each hour right before the BT connection goes to sleep:
This prevents your Android device from entering any deep sleep states but drains way less power than when display is always on.
If you have BT problems even with the display always on, you may have a hardware problem, environment problem or BT transmitter problem.