Refresh rate forced at 50hz with "Clone screen" on Windows 10

[Akumasama]

As the topic says, I've recently connected my 1080p Plasma HDTV to the HDMI2 port of my new Asus Strix 1070, together with my old main 1080p Display.
I've been experiencing some bizarre behaviour with the refresh rate though.

1) If I set mode to "extend desktop", signal is 1080p 60hz on both displays (as it should)
2) If I use each display one at a time, signal is again correctly at 1080p/60hz
3) If I set mode to "clone screen" then signal oddly becomes 1080p 50hz, for whatever reason. I can't change the res/refrate of the secondary display, but I can do it for the main one (I have 60, 59 and 50hz settings) but whatever I put there, the video output remains 50hz.

Now of course I would be glad to find solutions to this small issue (CRU maybe?), but even more than the solution I'd be very happy to learn what are the causes of this strange behaviour.

Thanks for your attention.