MacBook Pro 2015 15" with ASUS PB279Q 4K @ 60Hz
In order for your connected display to use DisplayPort 1.2 you also need a compatible cable. I'm not sure about the third-party cable you were using but I believe the Apple Thunderbolt cable only carries DisplayPort 1.1a signals. 

If you buy a third-party cable that's confirmed to support DisplayPort 1.2, 60Hz MST should work properly. The DisplayPort products database may help in that regard.