I saw nothing about bitrate? I can't help but thinking that the same bitrate would look better at 4k, or even 1080p.
I only know about this practically in regards to 10-bit anime being more compressible than 8-bit, but I assume the same logic applies.
But the equivalent in spatial resolution is encoding near-horizontal lines... which isn't a common situation (unlike gentle color gradients) and isn't a problem in the first place.
Banding can refer to all sorts of things that form bands, like fixed pattern noise from a camera sensor.
I cannot imagine how this could ever be true. Have you experienced it actually happening for the same content at the same encoding settings?
"bitrate" = file size / duration. There is no reason why increasing the resolution should ever make a video more compressible. This is completely different from the 10-bit situation you described, which takes place because you're storing data in a different format.
You could choose to encode a higher resolution copy of a video at a lower bitrate, but it's nothing an encoder will do by default, and it should always result in the higher resolution version looking worse.