This issue can be noticed by looking at the Bell Icon on the top-right corner of the screen, and noticing that Notifications are not being cleared on their own:
On Settings > Control Panel > General Settings, make sure that User Notifications Retention is set to 1 Day
, and click Save
Doing so should start getting the notifications cleaned up daily. Even if this setting is already set to 1 Day, click Save
so that the setting starts working correctly. After Saving once, this setting can be placed on the desired retention.