Seems like a battery issue. Probably the vibration draws too much current and the battery just drops its voltage. Have you tried changing the battery?

No, I didn't. Maybe it's not a bad idea. I read about the issue on the internet and it seems pretty common issue that sooner or later hits most of Instincts Solar gen 1.

I contacted Garmin support since I've seen that in some of the cases they offered generous discounts for newer models or even gave them for free, but in my case they offered me to pay almost a full price for refurbished Instinct Solar gen 1, that most probably would have the same issue after some time.