I’ll agree with Julien here - this is pretty shallow. Diamond drag is usually on the order of just a few thousandths. It’ll also depend on how you’re holding down material - if it’s double stick tape on the back, it can be uneven between the tape, if it’s wax, that can be sort of uneven as well. If I look at the top image and “follow the dots” across it in the reflection of the pegboard, there might be a bit of a crown. Looking at the most recent picture, the cut on the right of the drawing (which I’m assuming should go all the way top to bottom) indicates it’s a little higher in the middle in that direction.
Try this. Looking at the surface so you can see the reflection of the wall behind, push down gently on the material and see if it moves - you’ll be able to tell if the reflection changes, but you might not see the material itself move (black is always harder anyway). Another thing to try is to use a known good straightedge (a machinist ruler is good) in different directions across the material, shining a light from behind (without touching the material with the light) and see if you can see any light sneak under the edge. Everything you’re describing says the material isn’t flat.