I have a TBT3-UD1-83 dock plugged into an iMac 5K (2017) running High Sierra. My setup is a triple monitor configuration with 2 additional BenQ BL2711U 4K monitors. One of the BenQ is plugged direct into a TB3 port at the back of the iMac. The other BenQ is plugged into the dock’s DP++ port. The dock is connected to the second TB3 port at the back of the iMac.
Note that I also have an external USB 3.1 hard drive enclosure plugged into the second TB3 port of the dock.
For the most part everything works great. I can get a proper 4K/60Hz picture on the second BenQ via the dock. However I get an intermittent issue whereby the screen will go black (i.e. picture disappears) for a couple of seconds before returning. It’s like a momentary flicker that lasts for a couple of seconds or so. It happens intermittently - maybe once or twice a day.
I’ve swapped out the DP-to-DP cable but the issue reoccurs. Just in case it might matter, I have one of your 10 port USB3 hubs sitting on top of the dock.
Note also that I don’t have this issue with the first BenQ that is plugged directly into the first iMac TB3 port (via one of your USB-C to DP cables!).