It could be one of a few things. It all depends on how the engine is created, and I have to be honest I haven’t even looked at what engine it’s using.
Anyway, foliage is a very heavy effect for rendering and will pummel fillrate. Do you know what else hurts fillrate? Resolution and effects. So, usually when a game increases on resolution, effects are rendered lower res in a virtual screen space to save on fillrate, and guess what else gets the chop? Foliage. So it’s possible foliage was foaled back to hit a native 4k.
Now on a personal level I would rather it was dropped in resolution some what, but that’s just me.
However that’s not the only possibility. Their foliage is created using alpha masking, not alpha blending. Alpha masking is very dependant on your screen resolution, as it’s mask is created based on how many pixels it has to work with. Alpha masking is usually uglier than alpha blending, as it has a hard edge rather than a faded one (unless using alpha to coverage which is another thing entirely but then that needs good AA). But alpha masking is a lot quicker, even more so with regard to lighting rendering.
Anyway, alpha to coverage works based on resolution. But the higher you go in resolution, the closer to the overall foliage texture mask will render. Think of it like a circle, and if you run in a low res screen that circle will get thicker (because it has less pixels to work with), but if you use a larger resolution, that circle will get sharper and closer to its original form, and will look like it’s shrinking.
So what’s happening is, as you can see, the foliage IS being rendered... it’s there. Look again at the pics above and you can see little stumps of grass where it should be, it’s just not full and thick.
Now this is a stupid mistake to make because you can actually increase your mask threshold with distance and resolution to compensate for this. So that your foliage looks thick and wonderful over large distances but never different over resolutions. This is something we did 15 years ago... it’s not a new tricks.
Anyway...
The only other differences I can see are the usual “console cutbacks”, like reduced shadow res, penumbra differences, AF and AO etc. All you would expect on a late 2019 game.
So yeah... can this be fixed? He’s, because essentially it’s not a bug, it’s an over sight. Does this mean the X is doomed? No. It’s an oversight. Is the Pro using magicsouce to render all this grass? Read my post again, and look at the resolution difference.
In short, until somebody posts a native 4k shot from PC with the same Xbox settings, I can’t say for sure. But the above is a technical run down on who’s happening and why it happens, and yes, with a few minutes it can be fixed easily with no performance drain (might be a slight drop due to fillrate once the masks render in again however the foliage entities are already generated so that won’t hurt).
EDIT: Looking again, I can see some parts where the foliage isn't being rendered at all, so they are using a lower foliage density setting for sure in combination with the above. But you would expect that at high resolutions. Still, I see no reason why this can't be achieved on 4k, and until then will see it as a mistake on their part.