Hey, first of all, awesome library!
This particular combination of operations seems to result in faces going missing in preview mode:
difference() {
linear_extrude(35)
shell2d(2) polygon([
[15, 15],
[28, 15],
[28, 28],
[15, 28],
[15, 15]
]);
cube([20,20,20]);
}

I've highlighted the cube for clarity. The render turns out fine.
Hey, first of all, awesome library!
This particular combination of operations seems to result in faces going missing in preview mode:
I've highlighted the cube for clarity. The render turns out fine.