The Windows 10 Notification seems to be blocked when StrokesPlus is enabled. I've tried to trigger the Outlook notification as well as the Slack notification. Please see this example utilising the Slack notification preview:
Try unchecking the Keep Gesture Draw Window On Top* preference.
Some applications/Windows might think there's a full screen application active and somewhat recently they've suppressed notifications if this is the case. For example, when I play World of Warcraft I won't get notifications either.