I have the same issue, and it's actually pretty new for me. I needed a new DVR box to be able to view Peacock through Xfinity and the issue has existed with netflix SD shows since the new box arrived. Live SD channels present fine (unstretched with the black bars) as does Peacock and Hulu through my Xfinity box. However, I do not have the issue when watching through Netflix on AppleTV. I would love to resolve this as I know it has worked correctly in the past.
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