HDMI screen size

Mirroring forces you internal and external displays to use a common resolution. Sometimes one of the displays must change so much that it becomes unacceptable or drops out entirely.

Turn off Mirroring.

Hi,

Open System Preferences > Displays > adjust the settings to what you want.

Then take your Mac to an Apple-Authorized Service Provider, because no one else reports such a problem, so yours must be broken.