Not sure if FireTV Cube is on a different app version or not, but FireTV Stick 4K’s are having issues playing 4K HDR files. It’s being addressed (hopefully) in this thread:
I played 4K HDR files last week and they Direct Played fine, this week, they’re transcoding video, says Bitrate or Resolution limitation, but these files are files that played fine last week. So I think something in either the recent server update, or the most recent app update caused it.
The fact the OP is able to play transcoded 4K files leads me to believe (possibly) that his files are no longer HDR, they may be SDR since he/she transcoded (depends on the method used). But for sure, none of my HDR files are playing at all (Without transcoding), and all of my SDR content (still x265/HEVC) is playing without triggering transcode.
