There are a bunch of cable box models and Comcast is constantly fussing with things. They roll out changes in regions. We could each have options that are not available in other regions. I’m constantly being surprised by options that are new or ‘missing’ as I work with cable boxes.

I don’t think that you’ll have a CEC defeat option in the cable box. You could disable HDMI pin-13 or use an in-line adapter. After using a TV option to defeat CEC, make sure that all of the audio options that you need are still available.

I don't have an answer; but I do have a story.

I had a client who had a system that would pop on in the middle of the night. I did some stuff in their old house, and this was in their new place. While chatting over email with some basic level 1 troubleshooting, I said they had a "hodgepodge" of equipment (old receiver I didn't sell, TV they bought at Sam's, existing cat5 and ancient extenders running from cabinetry across the room to TV). For whatever reason, she flipped out when I pointed out the mix of gear, so we went our separate ways.

Bottom line, probably CEC / HDMI control somewhere lol.

She probably had carefully researched all the equipment she had bought, and had ended up with the absolute cheapest equipment, so when she heard you use the word "hodgepodge," she guiltily figured you were calling her cheap. Which she was. And knew it. And everything was okay until you said it out loud.