oh cmon, how could it be TV if it works fine on most channels? It says in manual, just scan = zero overscan. If particular channel has garbage data inside scan lines, call cable to fix it, unless it's analog, those had spec of 3-4% overscan and CC and other data was send in top or bottom scan lines, which then were off the screen and not visible, otherwise you would have that data showing as random noise, with TV set to 0 overscan it shows up. I'm surprised they still send analog signal anywhere, over the air broadcast is all digital for few yrs now. If the channels are digital and showing this, there is no excuse, just sloppy broadcasting.
Until they fix it, use any other setting except just scan, they all have overscan build in just to cover this prehistoric artifact, going back literally to very beginning of television.