Why Does the Odd Multiple of $ \frac{\pi}{4} $ on Gaussian Cause Loss in Repeatability Under Image Rotations?
The answer boils down to 2 issues with the practical approximations of the Gaussian Kernel:  


Though the Gaussian Kernel is radially symmetric its discrete approximation has a rectangle support. Unless this support will have infinite length a rotation by any angle different from a multiplication of 90 degrees will yield a shape which has to modified to fit a rectangle. Either choice (Cropping or extending) will yield different results as the weights will be different from the original approximation.
One must pay attention that the longer the support of the filter the less this issue is pronounced.
If I remember correctly, the SURF algorithm uses Box Blur based approximation of the Gaussian Kernel. This approximation, though can be very close visually, doesn't retain the radial symmetry of the Gaussian Blur hence even with large length ha different results when rotated.


I hope this clarifies the issue.