Hi, i have the same issue with my gtx 1060. 50" 4K TV, Sierra and Win10 dualboot. Win10 works great with hdmi 2.0 but sierra dont.
i tested the club 3D adapter and two other DP1.2 to HDMI 2.0 adapters from amazon.
All three adapters work in win10 perfect. in Sierra all 3 adapters are working perfect, but only 4K 30Hz. 4K and 50 59 or 60Hz is impossible to choose.
All 3 adapters use the same chip and all 3 adapters work with win10! But when i open the nvidia settings in windows 10, then i can see, that these three adapters have the same issue:
My GTX 1060 has 3 DP and 1 HDMI connections. As i plug in one of this adapters, then my pc says, that my GTX 1060 has 2 DP and 2 HDMI !!! connections. Because of that i think is it impossible to run sierra with 4K60Hz. Because it is impossible to run it with HDMI 2.0 and with this adapter i geta new HDMI 2.0 . My Displayport is no displayport as i plug in this adapter