Hi jkjaer
Read your post with interest.
i have basically the same system as you, core, bl20 and lg oled tv (c7). I also have exactly the same issue you describe with the optical cable connected to the tv. Almost always the core will switch to optical if the tv is off (within 10sec to 2min). No issue if I play music from any source while the tv is on, or unplug the optical cable from the core.
I have a second issue which only started about three weeks ago. Sometimes when the system is off, it is waking and playing a short burst of static (about 30sec and fairly loud). This is happened only three times over this period, with and without the optical cable plugged in. Ie the only connections the core has aside from the optical cable are the power, power link, Ethernet and our wireless network through which we play from Apple products/Deezer.
I don’t have the registration issue, but are using iOS.
be keen to hear if you have found a resolution, or experienced the second problem.