Just got my new 2017 LG OLED 65B6A and apple TV 4K. When I connect the Apple TV 4k to the Bose receiver, the picture keeps blinking on my TV. So when I connect my Apple TV directly to the LG TV, it is perfectly fine.
The input on the SoundTouch 300 is a pass-though connection, there should be no issues. However, what are the current video resolution settings on your Apple TV 4k? I would just like to gather this information so we can look into this for you. For the time being, I suggest to leave it plugged directly into the TV.