U4320Q, low contrast

Hello,

I'm quite happy with the U4320Q, but I'm not able to measure a contrast ratio anywhere close to the specified 1000:1. I've performed the measurement multiple times with a X-Rite i1Display Pro monitor calibration device and each time the contract was only around 400:1. The best I could achieve was 441:1 with the DDC/CI turned off as suggested in the X-Rite troubleshooting page:

https://www.xrite.com/service-support/poor_results_with_i1display_pro_on_pc

I've tried it with 3 different computers and the results were similar:

1. Lenovo Thinkpad T470s Win10Pro in Ultra Dock Pro using the DisplayPort and HDMI outputs. I also tried the USB-C output directly from the laptop.
2. Lenovo Thinkcentre M700 Tiny Win10Pro DisplayPort.
3. Generic Win7Pro PC with GeForce GTX 1060 DisplayPort

I've noticed at when the Thinkpad  boots the monitor starts in 3840x2160,60Hz 30-bit mode and once the Windows loads it changes to 3840x2160,60Hz 24-bit. While the monitor is in 30bit mode the background brightness is noticeably lower resulting a better contrast ratio. 

Using the Lagom digital camera based contrast measurement I was able to confirm the ~400:1 standard contrast and the somewhat better 676:1 contrast while the Thinkpad was booting

http://www.lagom.nl/lcd-test/contrast_ratio.php

Standard 399:1 contrast:
http://www.lagom.nl/lcd-test/contrast-view.php?id=14666

Boot 676:1 contrast
http://www.lagom.nl/lcd-test/contrast-view.php?id=14669

The only review I've found measured 673:1 contract which very close to the boot time value I got, but I'm not sure what hardware they used.

https://www.color-management-guide.com/dell-u4320q-4k-monitor-review.html

Is there any monitor or graphics adapter setting which would allow to use the better contrast mode in standard 24bit mode? I'm really puzzled by this.