XPS (9550) BIOS 1.1.19 has killed the HDMI link when monitor turned off

Hi all

Laptop is hooked up to 4k monitor. Since updating the BIOS to 1.1.19 last week, when I turn the monitor off and then back on again later (the laptop stays on but closed during the monitor being off) all the Windows have resized as if they were on the laptop screen.

I can only assume that the HDMI port is now assuming that 'monitor off' means 'switch to laptop display panel' rather than 'wait for instructions'(!), like me opening the lid.

Always worked fine before... switch monitor back on 12 hours after switching off, and the windows are all same size and in the same place.

Can Dell confirm whether the way the HDMI monitors activity was changed in the recent BIOS update?

Thanks :)