No 5.1 output in PS3 Plex App but fine through PS2 Video App

If I go through the PS3 “video” menu I can navigate to the Plex Server and if I play video files I get full surround sound OK.

However, if I go through the PS3 Plex app - although my receiver says it’s surround sound, I only get stereo sounds through the speakers.

PS connected via optical cable to Sony receiver. All other apps, such as Netflix play surround sound without issue.

Very frustrating, but I wonder if there is a setting somewhere I’m missing. I’ve attached logs. App.zip is going through the app, Video.zip is going through the ‘video’ menu.

Any help would be greatly appreciated.