2017 15" MBP & UHD TV: 4k60Hz *very* intermittent
I have the same problem, but with a MacBook 13" 2017, a Samsung 4K TV, and a USB-C to HDMI 2.0 active cable.

How to solve the problem:
First connect the USB-C end of the cable to the MacBook, then after a couple of seconds connect the HDMI end to the TV while powered on. I got 100% success in having 4K @ 60hz using this procedure.

How is this possible:
Not too technical, but... The active chip on the cable's HDMI end first powers up and initializes from connecting it to the USB-C port, and in sequence, the correct resolutions and refresh rates are offered as soon as the other end of the cable is connected to the TV.

I know this is not a proper solution, but at least it doesn't make me want to throw the MacBook out of the window!

I hope there's some more feedback from other users.