(Not sure if this is for plasma or kwin (or both), I'm using kwin)

At the time of this writing, Plasma shell (v 5.18.5 and below), by design, do not display any non-critical notifications to the user if any app is actively in full-screen. Apparently, this behaviour (https://community.kde.org/Plasma/Notifications) is intentional, and designed to prevent potentially-revealing/embarassing moments in a presentation (for example). Plasma leaves it to the app developer's discretion to set the notification urgency level, meaning, if the app dev sets the urgency level to 'Critical' their notifications come through even in full-screen.

Personally, I believe this behaviour should be configurable, meaning, User should be able to configure if they'd like to see all notifications even in full-screen (regardless of the app's notifyrc urgency value, maybe).

An instance where the current behaviour was troublesome for me, was, few weeks ago, I was working on a remote computer via VNC in full-screen and received important messages from a co-worker on Slack but these notifications weren't visible until I exited the full-screen because Slack's notifications are 'Normal' urgency.