If you aren’t seeing macOS notifications from Arc, the best first step is to confirm that you have enabled this option in your device’s System Settings. Simply head to the Apple icon > System Settings > Notifications then locate Arc in the list and toggle the option to “Allow Notifications” on. After that just restart Arc and you should start seeing macOS notifications from Arc.
However, if either notifications don’t start appearing or you already allowed notifications from Arc in your device’s System Settings, the best next step is to change the notification style to "alert" and then back to "banner" in your device’s System Settings. After a few notifications this should clear things up allowing you to set your notification style back to whatever works best for you.
To do this simply click Apple icon in the Mac menu bar > System Settings > Notifications and then change the notifications to "alert" in the notifications menu for Arc:
If, after doing both of these things, you’re still not seeing macOS notifications from Arc please submit a bug report in Arc - type “report bug” in your Command Bar (Command-T).