Xperia 1 V external monitor stuck at 1080p

[Mangodan2003]

I've been connecting my Xperia 1 V to a 4k monitor from time to time for a while but now see there is actually an official Sony page on the matter here : https://www.sony.com/electronics/support/articles/SX912901

The problem I have is it seems impossible to make the phone output at the monitors native 4k. It's only ever outputting 1080p.

This is even when not using screen mirroring, I.e. the monitor is showing something other than what the phone screen is.

This is the case both using a 4k usb-c hdmi adaptor and using a decent usb-c to usb-c cable.

How can we make the phone output in 4k?

[Mangodan2003]

Just tried desktop mode again and it turns out that although it looks really low res due to what it does with the wallpaper image, it's actually 4k. But it's useless because it's impossible to make things fill the screen or even move them around, despite enabling the options to allow doing so in developer options.