To fix this problem you have to take more calibration images with the checkerboard close to the edges of the image. It would also be good to take images of the checkerboard at varying distances.
In the images you have the checkerboard is mainly in the center, so that you do not have any points close to the top and the bottom of your field of view. As a result, you do not get a good estimate of the distortion coefficients.
I tried a simple experiment. I used only the left images to calibrate the left camera separately, using the Camera Calibrator app. Then I undistorted one of the images using the undistortImage function and setting the output view to 'full'. This is what I got:
The warping you see indicates that you need to have more data points to get a better estimate of the lens distortion. And that means taking more images. Generally, this is a good way to see if you have accurate calibration. Even if you have low reprojection errors, you may have problems like this, when you do not have good coverage of the field of view.