Hi @Luke,
I'm experiencing issue on my emby installation (Terramaster f2-210). I was on 4.7.6 and everything was fine, then I decided to move to 4.8.0.8 but this seems to broke playback capability (especially on 4k streams). So I moved back to 4.7.6, but still the issues remain, so I checked and found out that ffmpeg process is feasting on RAM, even when the Emby Server is stopped (see attached screenshot).
I also attached an emby server log: you can find a correct playback of 1080 stream (txt line 455), and a failed playback of a 4k one, previously working (txt line 891).
Hope this will help in solving the issue, thanks!
embyserver.txt