When notification history is enabled, and there are history items: Upon mouse-over the (notification) icon displays "No notifications or jobs". The icon also remains greyed out and does not indicate a count of historic notifications. Plasma 5.11.5 Frameworks 5.40.0 Qt 5.10.0
That number shows the number of active notifications. If you run notify-send some message -t 0 then the number will stay. Mixing in the history will conflict with that.
Hmm... Right, OK... I foolishly expected the notification to work in the same manner as it did in KDE4 :) Sigh...