Underrun of low heart rate is not always indicated. One day it does, next day it does not.

I am not sure whether I understand correctly, but if you are referring to the Abnormal Heart Rate Alert, then please be sure to read the article Setting an Abnormal Heart Rate Alert on a Garmin Watch | Garmin Customer Support

It tells explicitly that it goes off only when you are not active (more than 10 minutes without movements such as steps), so that may be the reason why it sometime works and sometimes not:

Many Garmin watches can notify you with an alert when your heart rate goes above or below a specified level when you are not active. Inactivity occurs after the watch does not detect movement such as steps for a minimum of 10 minutes. The type of alert depends on the options available on your watch. If your watch can vibrate and sound a tone for other notifications, then those options will be available for abnormal heart rate alerts as well. See your owner's manual for more information.

IMPORTANT: Garmin watches are not medical devices. Heart rate data is not intended to be used for medical purposes, nor is it intended to diagnose, treat, cure, or prevent any disease or condition. This feature only alerts you when your heart rate exceeds or drops below a certain number of beats per minute, as selected by the user, after a period of inactivity. This feature does not notify you of any potential heart condition and is not intended to treat or diagnose any medical condition or disease. Always defer to your health care provider for any heart-related issues.