that looks fairly regular. I see some slight rotation. I see some deviations in the grid, which may be due to plotting inaccuracy (rounding to whole pixels in the library, or bad subpixel rendering, which is likely).
usually, people fail to get samples in the corners of the view. that’s where distortion from the lens is worst, and that is where the distortion model, or rather, the numerical optimization, will introduce even worse distortion, if there are no points to constrain it.
if you want reliable results in a certain region, you need to calibrate on more than that region.
another matter: OpenCV’s calibrations always assume both a camera/lens and a lens distortion… i.e. they estimate both the focal length and the distortion.
for that (the focal length), they require perspective foreshortening. part of the calibration is to estimate the pose of the calibration pattern. that only works reliably with clearly evident perspective foreshortening. that means the pattern has to be presented at an angle relative to the optical axis, i.e. some out of plane rotation.
your setup shows a frontal/topdown view. no foreshortening. you might not even have a focal length, i.e. the lens is telecentric/orthographic.
you’ll probably have to formulate and optimize/solve your own distortion model.