Documentation says that R will be applied if given.

> For each observed point coordinate (u,v) the function computes: [...]

Actual behavior: giving a non-identity R appears to have no effect on the result.

The affected function appears to be cvUndistortPointsInternal and the involved variables are: matR (CvMat* parameter), _RR (local CvMat), RR (local C array used in the calculation).

related discussion including reproducer: https://forum.opencv.org/t/why-r-is-not-considered-in-cv2-undistortpoints/8483 and issue