poita said:
I first noticed the patchwork background of the front projection screen when viewing the film in 70mm, I haven’t seen the blu-ray, but I notice it every time in the cinema.

Here you go (a click-thru the website to the full 1920x1080 image):
Top Wall Papers website - 2001: A Space Odyssey - Dawn Of Man (1080p)

American Cinematographer: Front Projection for 2001: A Space Odyssey, by Herb A. Lightman wrote:

The surfacing material used for the giant screen was a special 3M fabric coated with very tiny mirrored beads of glass. … This special lenticular 3M material comes in rolls and an effort was made to surface the screen by mounting it in 100-foot strips. However, because of a slight variation in reflectivity between rolls, seams were frequently visible under projected light. An attempt to match strips exactly proved unsuccessful, so the material was finally torn into small, jagged, irregular shapes which were mounted in a “camouflage” mosaic, shape on top of shape, so that there was no longer any visible variation in reflectivity.

Would this suggest that the contrast of the prints was not high enough for the brightness to further obscure those pieces, as they saw it?