How to make HR abnormal alert works?

I've set the threshold of HR abnormal alerts to 100.

My today's 2h31m mountain running, my average HR is 166, max 196.

After that, I take the subway back home, sit over 40mins in subway remained still, HR remained between 103 to 115.

But, the HR abnormal alert never works.

Is there anyone could help to clarify how it works and how to make it work?

Thank you.