The default setting for notifications is to not communicate information regarding send offline agent alerts for hidden agents.
We see below the default setting during notification creation:
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If an existing notification requires notification of both hidden and non-hidden agentsan update to this field, travel to the Security Notifications page.
Scroll right on the grid until you see the Send Offline Alerts for For Hidden Agents column, then toggle the slider as desired.
The change will take effect immediately.