Notification messages from the time BleakClient is created until client.start_notify() is called are lost.

Bonded client/device cache the notification status.
When a client connects to bonded server they can be immediatly notifying and the server can send notification data immediatly.
If server sends notification message before client.start_notify() is called that notification message is never delivered to the notification callback on the client side.

Platform Windows 10Pro.
Bleak: 0.23.0 and 1.1.1