This function:

can only handle boards where all markers are upright.
That is because the line

doesn't just assume CCW order of the corner points, but also that outCorners[2] is to the right and below outCorners[0] by exactly sz distance units along both axes. For a rotated marker, this is not the case.

The solution is simple: compute the horizontal size of the marker (i.e. dst_sz.width) as the 2D distance between outCorners[0] and outCorners[1], and the vertical size (i.e., dst_sz.height) as the 2D distance between outCorners[1] and outCorners[2]. The rest of the logic already handles rotated markers correctly.

I have not implemented this in C++, but i have ported the function to Python and made this change, it works well. See here:
https://github.com/dcnieho/glassesTools/blob/0a03b5a2a6f6c093ef77c15decdc7bd3e570ef06/src/glassesTools/plane.py#L137, specifically line 166

Another note when working on this function anyway: the comments on line 128 and line 133 are non-sensical, 128 is for setting up the affine transform, and 133 should be something like // perform affine transformation (i.e. rotate marker).