I disagree. My diagram was very poor. Maybe this one will be more clear. There is no where on the black line in which you would see a dark part of the moon. The only parts of the moon which you would be able to see from that far away would be lit by the sun.
It wouldn't be 100%.
The only place from which you could see 100% of the moon lit by the sun in that diagram would be from directly below the sun, and that's assuming you could penetrate the light of the sun and see through the sun itself.
This is the same sort of geometric objection to being able to see 100% full moon from round earth. The alignment that that would require puts the earth in the path of the sun and causes an eclipse, so you can't see a 100% full moon. The moon has to be off axis, and even though that's only 0.52° and the moon may be 99.9% illuminated, there will still be a slight terminator due to that non-alignment.
Fine.
But that applies to flat earth too. Even in the extreme shown in your diagram, in which the moon is many times the altitude of the sun, anyone who is off axis from that alignment will be off axis from seeing the 100% full moon. The arrangement of earth-sun-moon is just in a different order, but it's the same geometric dilemma. You could probably come up with a theoretical distance for which a 32-mile wide moon can be above the sun such that you can find a spot on earth below the sun that is less than 0.52° off axis, but then that's going to be daytime for the observer. To be off axis and still on the night side of earth, and make that angle <0.52°, you're talking about a moon that's 500,000 - 600,000 miles higher than the sun. Really?
You can't have your geometry issues with round earth full moon impossibilities and casually ad hoc them away for flat earth too.