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: https://ibb.co/J3rKCVf
Strokes version: 2.8.6.4 Windows version: Windows 10 Pro 1903 x86-64 PC: ThinkPad X380 Yoga
Please let me know if you need any further information.
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.