Dell U2719DC not detecting USB-C display output from 2019 Macbook Pro

I've had a Dell U2719DC monitor for a year, connected to a couple of 2019 Macbook Pros through a single USB-C cable for both power and display. This was working flawlessly until a week ago when the max resolution dropped to 2048x1080 before failing to detect the USB-C input at all.

I first suspected the cable, so tried another Belkin USB-C cable that works through a docking station to another Dell monitor (U2719D) and also tried the Apple lightning cable. Both had similar results, no display output detected.

I've also tried 3 different Macbook Pros running different MacOS versions, included one that is fully updated with no success.

Dell has now sent me 2 replacement monitors with no change.

I've tried the monitor through the docking station with DisplayPort input and that works fine so I think the monitor is not the problem. When I try the original USB-C cable through the docking station, the screen flickered a few times, then no longer detected the output.

It seems like the original USB-C cable is failing, but if so, why does the Belkin USB-C cable not work when connected directly? Dell support told me that connecting a Macbook directly through USB-C isn't guaranteed and perhaps I just got lucky for a year.

Can anyone shed any light on this?