Chromebook display changing when connected to external display

[SomeITBloke]

Bit of a strange one, this week I have starting seeing the following.  When a Chromebook is connected to an external interactive display (4K) the Chromebooks internal display is matching the screens resolution, even though the device native resolution is 1920x1080, and everything goes tiny.  Also when connecting a 3:2 device to a 16:9 screen the Chromebook display matches the external display, so you get black bars top and bottom of the internal 3:2 display, rather that black bar on the external display.  It's like there has been an update to ChromeOS.  I might have missed something but this is different behaviour the previously.