Those are compression artifacts. Assuming you took that from a screenshot captured via the box, that's not an accurate representation of what's being displayed. Screenshots are captured as JPEGs by default.
In this situation this is the Roku Home screen, not my program, so it's not 720p to 1080p conversion. The overscan, if happening, wouldn't smudge the graphics nearly that much. Just resizing the image 2% in any image editor on quick/dirty settings looks much better than the Roku result.
I believe it's image compression, pure and simple.
All of them have (except old tv) have sharp clear pixel-accurate lines in-game. It's ONLY the Roku Home screen icon that looks like mud, and it looks that way on all of them. That has led me to believe there is some under-the-hood shenanigans going on, such as Roku heavily compressing all the home screen icons for a performance boost, or a strange combination of resizing back and forth, or something.
(Interestingly, on this computer monitor, I found a setting for the HDMI input to act either as "Video" or "PC" - when I put it on "Video" it slightly zooms in the screen, cutting off the edges a little bit, creating overscan! Of course I set that junk back to "PC" where it will remain forever - where it was before.)
Back on subject...
So in-game there is no overscan happening at all. Is it possible there is some overscan happening only on the Roku home page? Maybe now you want to spill the beans how you measured 290x218? 😄
13.6cm/88.4cm*1920 = ~295
10.3cm/49.6cm*1080 = ~224
Maybe there is some crapification of side-loaded icon image files that doesn't happen the same way with private/public channels.
Maybe they get squished, prodded, translated to jpg, enlarged, pushed into a box and shrunk down again, and compressed to boot.