As others have reported it seems that 5.3.1 fixes this. Over the course of this morning and afternoon I have had the TV and Xbox on in the background and can report that:
- BBC 1 HD played without issues - for over 1h 30m until I changed channel.
- BBC 2 (Standard Definition) played without issues - for over 30 minutes.
- ITV 1 (Standard Definition) played without issues - for nearly an hour.
- 4 HD played without issues - for just over 2h 30m.
Whilst BBC 1 HD (Channel 101) is good I am still having issues with the standard definition version of BBC 1 (Channel 1) here.
The Samsung Tizen preview running on my The Frame 2019 didn’t have any issues with the standard definition version of BBC1 in 5.2.1 but it looks like there may be an issue with audio as that channel appear silent now 
Update:
@Moussa: I got to the bottom of both the XBox preview giving me the “An unexpected playback problem occurred” error and the Samsung Tizen preview having no audio when trying to tune Live TV to BBC1 (Channel 001) on Freeview.
For some reason both apps are showing the following available audio tracks for this channel as:
- English (MP2 Stereo)
- English (MP2 Mono)
- Unknown (MP2 Stereo)
…and unfortunately both apps are selecting the “Unknown (MP2 Stereo)” audio track despite having the application set to either “English” or “English (UK)” in Settings → General → Language.
If I start viewing the channel, allow it to start buffering and then change the audio track to “English (MP2 Stereo)” from the popup display then on the Xbox app it plays, displaying video and playing audio and on the Samsung Tizen app, which was already displaying video, then I also have sound 
Unfortunately as soon as I navigate to another part of Plex and return to viewing Live BBC1 then I have to set the audio again to “English (MP2 Stereo)” as it will automatically chooses “Unknown (MP2 Stereo)” otherwise Plex will fail to play the channel resulting in the above mentioned error message again.
