Ok, I am replying to my post about this because I have fixed my issue.
The way I fixed it was buying an ACTIVE mDP to HDMI adapter (one that supports 4K @ 60Hz) and using that with my monitor. I have had no luck using the direct mDP to full DP cable.
Thanks for sharing, this solved my issue as well. Monitor sleeps like a baby now.
For reference here are the adapter and cable I'm using:
Adapter: https://www.amazon.com/gp/product/B00PJ3LSIG/ref=oh\_aui\_detailpage\_o00\_s00?ie=UTF8&psc=1
HDMI Cable: https://www.amazon.com/gp/product/B014I8SIJY/ref=oh\_aui\_detailpage\_o00\_s00?ie=UTF8&psc=1
Wow I can't believe I finally found people that were having the same issue as me!! I checked so many forums without success.
Fortunately my monitor also has HDMI, so I will order this active adapter asap! I had been plugging the second monitor directly into the SP4 so that I didn't have to manually turn off the monitor for over a year now and it is starting to wear out the connector.
It flickers on and off if the cable moves now, so I was frantically looking again for a solution.
This is exciting, I hope it works. I believe last year when I called Microsoft about this they made sure I was using DP input! Geez! I can't believe Microsoft is still clueless. I am glad I didn't waste my time going to the Microsoft store. Now I just need
to figure out whether to keep the 32" 4k or the 34" ultrawide since I just bought different monitor to see if it would work properly. (it didn't)
Thanks for posting this!! If it works I will post an update!
I used the above mentioned Cable Matters active adapter:
https://www.amazon.com/gp/product/B00PJ3LSIG/ref=oh\_aui\_detailpage\_o00\_s00?ie=UTF8&psc=1
Now my LG 34UC88-B sleeps like a baby when connected through the Surface Dock. Unlike when using pure displayport connections (as recommended by Microsoft).
It is now fixed and I am happy, but I can't believe Microsoft couldn't figure this out within 3+ years it has been out!! And if they did, a simple TSB advising of the fix would have gone a long way!
But Microsoft still needs to fix the Dock to work with pure DisplayPort as others have mentioned monitors that cannot reach full capabilities through HDMI. (The 32" 4k I mentioned above is one of them, hence I am returning it)