The box is first sensing, via the HDMI handshake, that the TV is only able to handle stereo so the cable box is only outputting a stereo signal to your Pioneer. However your Pioneer has a standby feature so you should run only one HDMI to the Pioneer and one HDMI out of the Pioneer to the TV ARC input. If the Pioneer is in standby mode you will be able to listen to the TV speakers. Then when you want to listen to surround sound you would turn on the Pioneer using the same single input on the Pioneer.
This is correct. This is how it worked with frontier.
With Comcast. With a single HDMI from Comcast to pioneer and then the one master HDMI out from pioneer to the tv I only get audio when the receiver is in standby. You're right, normally when I turn the receiver on (at least with frontier) audio was coming through the receiver now it's not. I've changed the audio settings in Comcast many times to auto detect.
I'm not noticing a separate tv in our bedroom with our second Comcast box keeps losing audio. This is an old insignia tv. To fix audio I have to go to Comcast settings. Switch to auto detect and then switch back to stereo and then sound comes back.
So now both cable boxes are giving me audio issues and their customer support is useless.