You're not supposed to see the eye after adjusting the brightness. With an OLED, the blacks are so low that it's difficult to see where it really completely disappears. If you put the console at standard(16-235) and the C9 on high (0-255) you'll see that the closed eye is the same as the first box on the scale which is outside the tick marks. The tick marks are at 16 and 235. Here's what I'm talking about with standard/high so you can see that it's outside the 16-235 range:If you use pc black level for the Xbox you cannot see the eye on the Xbox calibration - so use standard level and low.
If you look at the sun screen you'll see that it's also outside the tick marks on the colour scale. But here they put a warning that it might not be visible on some TVs. Ironically the C9 doesn't clip at 235 and does show the sun (when using standard/low).
So if you don't want to adjust your set outside of the recommended settings, don't try to get it to show the closed eye. Just make sure you can see all the boxes between the tick marks. If you're running standard/low then the sun will be visible and if you're running PC/high it won't. Both those are OK.
I think this is a result of the C9 (and other OLEDs) super low black level. There's just no room to show "blacker than black" signal levels without cranking the brightness way out of calibration.