The answer is correct from Samsung & Onkyo.
The problem is likely in the Onkyo since that's the product which has to deal with the HDCP and EDID and the splitting of the HDMI source.
The problem is though that the TV in the bedroom is likely the culprit which is messing everything up. It may not conform to proper HDCP or EDID specifications, and it may be causing an override in the output of the Onkyo receiver since it is not a 1080p display. It may have a preferred native resolution of 1280x720 instead of 1920x1080 which is messing everything up. The Onkyo receiver may not be able to properly handle the two preferred resolutions properly... etc., etc., etc.
You can look for an inexpensive scaler to put between the Onkyo and the bedroom TV that can convert 1080p to 720p. I'm not sure an 'inexpensive' one can be found, and it may not truly fix the issue.
- Note, one of the features specifically is down-conversion for multi-resolution distributed display setups.
I don't believe it is your cables.
Can you set the output resolution of the Onkyo to 720p? Can you force it to that?
Have you updated firmware on the Onkyo to the latest and greatest?
I'm out of other ideas at the moment.