During last summers Olympics while Dish was replaying events in 4k I had that same result. I had my Dish / 4k setup on a splitter with 2 other HD sets. This caused H3 to down rez 4k to 1080i. Apparently, HDCP causes H3 to do this because of copy protection. Once I disconnected H3 from the splitter & plugged the HDMI directly into my 4k set it displayed true UHD.My 55E6P OLED displays 4k, but says input is 1080i.
I can't go through powered HDMI 4K splitter either. Its loses the 4K signal with hopper3. I don't have same issue with Roku or Firetv box the splitter works with 4K.