It's your TV. The overscan is normal on HDTV's receiving what is in a broadcast TV video format, YCbCr. The TV set is what converts that signal to RGB. It may allow underscan, or zero overscan, when presented with an RGB signal from a PC, or a device with that type of format.
You'll find no problem with this with even an entry level 4K set. You can find a workaround with a different device that outputs an RGB signal to your HDTV. Or you can put in a feature request to a manufacturer to add RGB output over HDMI not for compatibility, but to "enhance" the viewing area by 1 or 2 % on what are essentially obsolete displays, while providing no advantage to current and future 4K sets when HDMI bandwidth can be conserved by using YCbCr with 4:2:2 or 4:2:0 sampling over the uncompressed 4:4:4 or RGB formats, especially at 30 and 36 bit depths.
Good luck with that last one, though.
How can it be the tv when the problem is intermittent, on each separate Roku whilst no such issue with Apple TV or Firestick.
No, it's not the tv.
