note the CheckBox3 H x W were incorrectly set to pixels from Automatic while in the Horizontal layout. Especially discerning in the erroneous H and W do not change back to Automatic on returning to the Phone display.
It seems to be the case that this only happens if you have a component other than the Screen selected at the time you switch the size. It looks like whatever component is selected is taking on the width/height of the screen when the switch is done. As a temporary workaround, you can select the screen prior to changing the dropdown.