Yeah that makes sense. My TV doesn’t allow user control to scale down to pillarbox or zoom out, only to upscale to wide/zoom in/panoramic. I don’t see overscan when I play direct over component to the TV or from the Xbox 360 @480p over HDMI to the TV, so I wonder what is making the TV mishandle the OSSC signal.
Different input types are often handled differently by TVs; so your component input is likely doing different processing to what your HDMI inputs do. For example, I have an older Samsung LCD that will absolutely do 240p correctly over YPbPr component, but not over composite or HDMI (And probably not RGBHV, but I haven’t tested that one).
I also have experience with a Panasonic plasma TV from about a decade and a half ago, which has an HDMI input, but it forces overscan on it; even though it has aspect ratio adjustments, disabling overscan didn’t appear possible. You might be having a similar issue, and I would suggest eventually getting a different display that would allow you to properly display inputs without overscan.