Why does my external monitor keep waking up when MacBook Pro is asleep?

Hello Tim baker,

It sounds like after your MacBook Pro is put to sleep your external display will randomly turn on and show the login screen.

"If your Mac wakes unexpectedly

Your Mac might wake for these activities:

• Certain network activities such as those related to music sharing, photo sharing, printer sharing, or file sharing.
• Enhanced notifications from FaceTime, Messages, and other apps and services.
• Activity from Bluetooth devices that are connected to your Mac. To prevent Bluetooth devices from waking your Mac, choose Apple menu > System Preferences, then click Bluetooth. Click the Advanced Button, then deselect ”Allow Bluetooth devices to wake this computer.”

Some Mac notebook computers are designed to start up when you open their lids or connect them to power."

Check out this link for more support when a Mac doesn't wake or sleep as expected.

If your Mac doesn't sleep or wake when expected

Cheers!