Some cameras (nearly all if not all) display this behavior: when a raw image is magnified, it becomes blotchy and blocky at 100%. That’s because the camera is not rendering the raw file, but magnifying the low quality heavily compressed JPEG embedded in the raw file, which makes the image blotchy as well as showing the square JPEG tiles—not good at all for checking image detail or critical focus and a real waste of the high-res Nikon D850 rear LCD.
With the Nikon D850, I see a night-and-day difference in image quality when zoomed into 100% or 200%: with Raw + JPEG Superfine, the image is smooth and detailed, free of blotchiness and the obvious JPEG blocks seen without it. The Nikon D850 is clearly displaying the high quality JPEG, not the low quality JPEG embedded in the raw file.
Other cameras, Nikon or other brands (like Ricoh GR), might benefit from this approach.
So if reviewing images critically in the field is in your shooting approach, be sure to set the camera to RAW+JPEG (I chose the highest possible quality for the JPEGs).
When I download my images, I just select all the JPEGs and trash them. This is a nuisance, but it works. Another solution might be to have the camera record the JPEGs to the 2nd storage card (TBD: I need to verify that this works).