I am starting to add 4K UHD (w/ or w/out HDR) titles to my library as I own a few 4K HDR TVs, tablets, & phones. However, the majority of my streaming devices are still max. 1080p or less. When attempting to playback my 4K titles, my 720p 32" TCL RokuTVs initiate playback (green progress bar on device completes and playback shows in my server Dashboard) and then show a blank screen: no picture, no sound. I can fast-forward and rewind and the video thumbnails appear to function normally. My server hardware is fairly robust and appears to be transcoding video from h.265/HEVC to h.264/AVC and audio from 5.1 AAC to 5.1 AC3 as expected. I've tested this with both Emby and Emby Beta apps on the TV. HEVC 60p playback was disabled by default, but frame rate of movie is 24p, like all my titles, so I can't see that mattering as non-UHD movies play without issue.
I was researching this in the forums extensively this morning and found what may or may not be a related issue on the Fire Stick/TV.
I checked my logs (attached) and found that the transcoder was not stepping down the resolution of the video stream from 2160 to 720. I then went and tested other videos and found that video stream with a resolution of 720p or less played fine, but anything 1080p or greater experienced the exact issue described above, to include 1080p videos encoded w/ h.264/AVC video and 5.1 AC3 audio streams that show as "Direct Playing" on the Dashboard while the device screen remains blank.