Your second plot is essentially a one-dimensional array. Try the code without the second coordinates.
fig, axs = plt.subplots(1,2, figsize=(20, 10))
axs[0].set_title('Undistorted Image')
axs[0].imshow(undistort_img)
axs[1].set_title('Warped Image')
axs[1].imshow(warped_img)
