Push Notification Issue

After closing the tab, I am unable to receive any push notifications from the site.

Site: https://calendar.protonmail.com/

I receive the notifications when the site is open in the browser but even when I am surfing the net and the site is closed (here I mean that the browser is running or is in use but the site’s tab is closed).

This issue is not particular to the site, even when I checked it through a push notification checker, the same thing happened. After closing it’s tab I did not receive the notification.

Please look into this matter