The problem I experience affects my TB3 dock, USB-C displayport direct to a USB-C monitor and through the Lenovo travel USB-C hub. The display will start flashing in and out and go to sleep. Once there is a malfunction, the display will not come up, all ports on the dock stop working except the sleep/wake button, disabling cellular won't help until a shutdown and boot (warm restarts often did not help). The dock will charge my tablet in this state just fine, and disconnecting/power cycling the dock makes no difference.
Do you have the cellular WWAN option? It took my a while to figure out that my problem is caused by the cellular modem. I speculate that there is radio interference that affects the high-speed TB3 & displayport on USB-C. If cellular is enabled, the TB3 dock may come up, but then it will quit working at various intervals, sometimes immediately.
Lenovo replaced the mainboard on my device 3 times, and I could reproduce the issue for the onsite technician immediately.
The tablet has been replaced twice. The last machine they sent worked great for about 3 months, until last September and the problem started again. You can imagine how many hours I have wasted on this machine. I have laptop with USB-C TB3, and it works perfectly with the Lenovo dock every time - never an issue. Though they also replaced it as well.
Now I suspect that a software update has caused the problem to come back, I want to try bringing the device and firmware back to the factory image and older firmware to see if the problem goes away, but this will take many hours to test and I really don't have the time. Though it shouldn't matter, I also want to try a different cellular carrier.
Be sure to update your TB3 dock firmware if you haven't - Lenovo vantage did not do this automatically, I had to go to the support page and manually run the update. It was running very old firmware. I am not sure if it helped, but it will save time dealing with support.
Good luck!