I have no experience of this, but I came across something about this while looking for info on 5.1 passthrough for the same TV. Here's what you need, I believe: http://www.eurogamer.net/articles/digitalfoundry-2016-best-settings-for-hdr-on-samsung-4k-tvs
Basically you have to manually enable UHD on each HDMI port or it won't display it. Hope this helps....