I had contact with Edision, they solved the issue on the HD channels such as ITV HD , CH4 HD, CH5 HD, BBC HD... For the DVB subtitles only. This bug is overcome in firmware version 1.04.68 (I am testing it right now!)
From my experience I can say that the issue for the Teletext subtitles isn't fixed. Edision tells me that it will be fixed in the next firmware update.
I hope they can fix the issue with the dissapearing subtitles when you watch Timeshifted. They informed me that they are working on it!