Hey Ben
Any USB-C dock should work. We don't have the exact one you have (with dual HDMI outputs), so I picked one up - should arrive tomorrow and I'll test it again! Its possible that multiple monitor outputs is causing something unusual to happen...
What power supply are you using to power the monitor and the tachyon out of reference?
For the USB-C cable, is this a USB 3.1 cable? I ask as it needs the specific display port alt mode and high speed signal support to work correctly - we had to sell such a cable in the end as there was such high variability in these within the company (and confusion!).
Lastly, what's the device ID if you don't mind sharing? I'll check some logs. If it's possible to leave this device online so they can upload, that would be amazing.
Thanks
Nick.