When you add a resource to a project, you receive a notification in the Classic UX (Organizer), but the same notification is not seen in the Modern UX notifications.
Release : 16.0.3
This notification is sent as part of the setting: Admin > Project Management > Settings > Automatically Add Staff Members As Investment Participant > When Added to Investment.
However, Participants are not part of the Modern UX.
This is working as designed. As Participants are not part of the Modern UX, the notification is not visible in Modern UX.
The functionality to notify when a user is added to an investment (not tied to a participant) can be requested as an Enhancement: