Like others in this thread, I am experiencing the same issues as the OP. My Bose sound bar is not detected when I turn on my Roku TV. The sound bar needs to be powered on before my TV can detect it, which does not happen when I power on the TV and the eArc then powers on the sound bar. I have to go into the CEC settings and manually detect the sound bar. I have played around with every possible audio/hdmi setting to resolve the issue. I updated the software on my Bose sound bar. I upgraded my HDMI cable. The only solution is to disable the "system standby" and leave the sound bar always on so that the TV powers on and detects the (already active) sound bar. (Additionally I experience a lag unless the Roku specifically detects it as "sound bar" and not a "speaker" as shown by the volume icon.)
Really hope Roku pushes an update so that the TV detects CEC devices after it powers them on. I bought this Bose sound bar specially because it was "Roku Ready", and it has yet to work seamlessly with my Roku TV.