I am late to the party here and have found this this is the best (or most depressing if you like) thread about this topic. I have all the symptoms. I am experiencing them as I scroll down this thread and I am using the Version 21.20.16.4678 video driver.
A couple of point on trouble shooting. I've had my surface for many months, but have only recently used it in a docked configuration with an external monitor recently with great frustration.
At home I have a 4K monitor where the problem is most prevalent. At work I have a curved monitor and it only has an issue occasionally (generally in video calls such as Webex). At home the issue is while scrolling or very basic screen functions.
My wife uses my old SP3 in the same docked configuration, no issues. My son has an early SP4 and has no issues. My SP4.2017 is the only one with issues.
I've tried the old original surface dock and the newer clock style ones. Same results.
Connect the cable externally. Same result.
Leave the keyboard open. Same result.
Have lower resolutions on the external screen. Same result.
Change the refresh rates. Same result.
I am going to formally create a case with Microsoft.