U2419HC, USB-C connection to Thunderbolt 4 dock problems

Hi,

I'm having problems with my my Dell U2419HC monitor which is connected to a ThinkPad Universal Thunderbolt 4 dock via a USB-C cable. The dock then connects  to my ThinkPad's Thunderbolt 4 port via a Thunderbolt 4 cable. 

Before I start with the problem, I have the same model but different versions of the ThinkPad which I use as a personal laptop (9th Gen X1 Carbon) and a work laptop  (7th Gen X1 Carbon).

My work ThinkPad connects flawlessly with the dock and the monitor. 

The problem is when my personal ThinkPad is connected to the dock after about 30 seconds  the screen brightness fades to black and then monitor loses the connection. pressing the keyboard or the mouse wakes up the monitor to a login screen.

I’ve never seen anything like it. The brightness of the monitor quickly fading is truly bizarre.

Here’s a video of the issue which starts around the 10 second mark. This happens every 10 seconds, making the docked setup unusable. 

Imgur video link

I’ve installed the latest Dell monitor drivers and firmware, factory reset the monitor, and nothing is attached to the monitor beside a power and USB-C cable. 

The only thing attached to the dock is the supplied 135W power adapter and the supplied Thunderbolt 4 cable.

I’ve tried a number of USB-C cables which are 3.2 Gen 2 cables which are capable of 10Gbps data, 20V/5A 100W power, and 4K video. I’ve also tried the USB-C cable supplied with the monitor and an Apple Thunderbolt 3 cable. 

I don't know why it works with a 7th Gen X1 Carbon and not my 9th Gen X1 Carbon? 

I also had the ThinkPad USB-C Dock Gen 2 which worked with the monitor with no issues at all. 

Any help would be greatly appreciated. 

Thank you