3 weeks ago my Edge 130 Plus started to do a strange thing: during (MTB) ride, sometimes stop showing the actual HR. After coming home, I thought that the HR sensor's battery is low so I replaced it. What surprised me, when I switched the Edge130+ next time, a popup came in about that, the device found a new HR sensor. That clarified me that, actually the Edge 130+ 'forgot' the HR sensor connection.
Next time when the HR field started to show '--' I just went to the Sensors menu and paired the HR sensor again. But this is quite annoying...
I'm using the same HR sensor with my Edge830, sometimes with my old Edge520 and also with my Garmin Enduro and I've never seen this issue with those devices. My Edge 130+ has the newest Firmware anyway (2.90).