I actually swapped the now working dock with the original user's laptop again and its still working. SO after a day or two, i was about to call Microsoft to warranty replace until I thought i'd dock it one more time to try again. The 2nd user this happened to, i just unplugged the ethernet so they can still work on the wireless for the time being.
everything with the computer's network setting, even though the computer is not the issue. Ran alot of different network command lines suggested by the internet to flush, reset, etc. I installed and ran the entire MS diagnostic kit, which didn't fix or diagnose anything that helped. I have the dock firmware updater install on all machine so they auto update the dock when firmware is available so i know it's not a firmware thing. You'd think it's a bad dock right? Well coincidently that same week, another user's dock was having the same issue. Tried three different surface books on the 'bad' dock and ethernet wasn't working. Took the 'bad' dock back for some testing. Once I discovered that the machine was on WiFi, I grabbed a different dock and ethernet is working. Interestingly enough, i've ran (and am still running into) this issue as well, but with the regular surface dock (gen 1) - I've done alot of troubleshooting myself and couldn't find an answer as well.
