OK, I guess that's because...
1. Your status is Busy and you have "Show Messenger Plus! popup notifications when Busy" unchecked (in Preferences\Customize!\Evetns Notifications)
2. You have EnableMsgPlusToasts set to 0 in your preferences registry.
Anyway, if I get things right, I may find a way to correct it...(hope the oldish way used in Hopper3 still works
)