Why do notifications disappear when I mirror a virtual screen?

macOS can suppress notifications while the display is mirrored or shared. Check this setting if notifications stop when you enable mirroring.

Open System Settings > Notifications. Under Notification Center, set when mirroring or sharing the display to Allow Notifications. Apple describes this control in Change Notifications settings on Mac.

This is a system notification setting, so allowing notifications can also make them visible during presentations or screen sharing. It does not create an exception only for BetterDisplay virtual screens. If one app still does not show notifications, check its individual notification settings as well.