Thank you for your bug report, I can confirm that with a docked laptop using the laptop screen and an external monitor, on user change the screen is in mirror, reconnecting the external monitor triggers an update and put the greeter in the right mode
Thank you for your bug report, I can confirm that with a docked laptop using the laptop screen and an external monitor, on user change the screen is in mirror, reconnecting the external monitor triggers an update and put the greeter in the right mode