How to get an LG Ultra HD (27UD88) and Macbook Pro 13" 2017 running at 60hz?
I got it working. I'm not sure exactly how, but here's what I tried, in the order of what I believe fixed it:


Enable DisplayPort 1.2 in screen configuration
Reverse the USB-C cable
Set "Ratio" to "1:1" in screen configuration (I've since tested this and switching it back had no negative effect)


(3) was recommended by a user in a forum thread.

Update 2018-01-29

I've since experienced the problem once more, without changing any settings. Attaching/detaching the cable fixed it.

Update 2018-05-06

After the most recent MacOS update (10.13.4) it no longer downgrades to 30hz, instead, it never comes on. If the cable is dettached, the computer crashes and has to be booted. So far, the only solution I know of is restarting the computer, which works 50 % of the time, and if not, reattach the cable (it usually won't crash after a restart).

This happens after screen sleep and detaching/attaching.

Update 2018-12-13

In this third evolution of the problem (MacOS 10.14.1) the computer no longer crashes. Roughly 9/10 times it will display in 30hz/horrible resolution and all I have to do (sigh) is detach/attach the cable until I hit that 1/10 where it works. It's a daily 5 minutes routine for me.

This setup (MacBook Pro 13" 2017 w/o touch + 27UD88-W) is not recommendable to anyone.