Hi @DRES
Thanks for the additional details. I suspected it might have been a Viiiiva HR monitor, which is known to behave oddly if you change its Bluetooth ID name.
Please have a look at the Heart Rate Monitor pairing page, specifically the part about the 4iiii’s default name.
You might also consider flashing new firmware to the HRM for good measure. There was a recent thread where another Viiiiva owner’s issues were resolved with the above two steps.