More investigation shows that this is actually a Unity bug. The black screen you are seeing is the lock screen but Unity doesn't have a lock screen when in guest mode. The lock screen is triggered when you switch users via the indicator.
You can also trigger this by typing alt+ctrl+L inside the guest session. It will show a black screen that disappears when you move the mouse.
The solution should be that Unity does nothing when the lock screen is requested to be activated.
More investigation shows that this is actually a Unity bug. The black screen you are seeing is the lock screen but Unity doesn't have a lock screen when in guest mode. The lock screen is triggered when you switch users via the indicator.
You can also trigger this by typing alt+ctrl+L inside the guest session. It will show a black screen that disappears when you move the mouse.
The solution should be that Unity does nothing when the lock screen is requested to be activated.