My daughter is having a similar issue. She bought a Garmin Forerunner 35 in November 2020. At the time she was connected via Bluetooth to a Samsung S9 (Android phone ). The phone will connect and disconnect constantly even when the phone is right next to the watch. She has recently bought the iPhone XR and she is having the same problem
This post might help. ForeRunner 35 WILL NOT CONNECT - Forerunner 30/35 - Running/Multisport - Garmin Forums