This issue used to come up from time to time on all Garmin watches I've had over the years. A soft reset (rebooting the watch by long-pressing the button in the top-left corner) usually helped for a while, but not anymore with FR 265.
I'm now on FW 18.22, and whatever I do (reboot the watch, soft-reset the watch, change the strap, change the connection of the strap from ANT+ to BLE) - nothing helps. The watch uses OHR to record HR data. This is obvious during Strength Rowing activities where I'm getting readings in 60-80 range (probably, the OHR is unable deal with wrist movement). The OHR continues flashing after the start of the activity too - not a good sign.
I have to manually turn off OHR each time before I start any activity now. It's not that difficult, but unnecessary. Thanks for help