I regularly miss calls because my Garmin watch only vibrates once when a call comes in on my phone. That is not how it used to work; vibration used to be continuous.
Clearly the approximately two years old change is in Connect Mobile, because the vibration is also too short on older devices that haven't had new firmware for over two years.
Several forum threads have been started by users of Forerunner 935, Fenix 6 (1), Fenix 6 (2), Fenix 6 (3), Fenix 6 (4) and Fenix 7 requesting that incoming calls vibrate continuously instead of only once. Even a petition has been started be user Pmozzati.
Although a workaround using Tasker (see this thread) is possible, this is far from ideal.
If you would also like to see longer duration vibrate alerts for incoming calls, please request it here: https://www.garmin.com/en-US/forms/ideas/
And siging Pmozzati's petition may also help.
Thanks!