I have the UDP-205 connected to a Sony 65x9005b TV which i had setup with ARC so the TV could play audio through the hifi, and CEC for convenience of control - and had never had an issue.
I recently added an Nvidia Shield to the HDMI input of the Oppo which runs Kodi / Plex to play stuff locally on my network (this is entirely to circumvent Cinavia).
What i noticed was that with passthrough enabled for a TrueHD 7.1 audio track the Nvidia was tripping out and sending LPCM 2.0 with no audio, however the 5.1 DTS or Stereo tracks would work with no problem.
Disabling passthrough on the Kodi or Plex client resolved the issue, but it was clearly the Shield doing the decoding and passing LPCM to the Oppo.
Fine in theory as should be the same, but it was annoying me the Oppo wouldn't decide the TrueHD source, and it seemed silly to just be using it as a pre-amp!
What fixed the issue was sticking the HDMI CEC setting on the Oppo to Limited rather than On - and suddenly everything works as it should with passthrough enabled! Et Voila!
The only downside is that the Oppo doesn't automatically turn on/off anymore when the Shield is turned off, and i can't find a way to get the receiver IR setup correctly with it.