Just the clarify you are playing the same 1080 file on both oppo and infuse and having issues and not the UHD. Did you make the HD copy yourself? Did you try having ATV plugged in to same HDMI port on the TV? There could be some settings there that are different too.
Oppo plays the exact same file as Infuse. Both are routed through an av receiver. Receiver video scaling turned off. Only difference is appletv output 4k resolution and oppo outputs 1080p on the Gangs of London file. 4K on akira uhd bluray.
So output should be the same. TV is calibrated as well.
It seems like you may be playing and SDR video with the output set to HDR.
Can you check to ensure the Match Content options are enabled in Apple TV > Settings > Video and Audio.
It is set correctly. I am getting an sdr signal according to my avr.
Would you be able to send in a sample we can review here?
I have uploaded an episode.