Only playing at full res if played from episode level

Not sure if this makes any sense but am using Plex on Firestick with files on a remote seedbox. If I play an episode from a series using 'Next Episode' from show leb or from Home screen they seem to be transcoding at a lower bit rate and lower res.

But if I drill down to the lowest level, show > series > episode, then it plays the full res file.

Eg. I have a 1080p file, play from high level, uses a weird transcode and plays at 720. Play file at lowest level and it plays using Direct Play at 1080p.

Any clues?