I am not worried at all about this issue and it will never show up in prints made(largest print I do is A3+) but I am interested in a theoretical explanation why this phenomenon appears. I have never seen this with other Canon lenses I have used.
Hi,
The phenomenon is aliasing artifacts. They are a result of recording finer detail than the sensor can resolve, and it's more prominent on cameras without an Optical Low-Pass Filter (OLPF or anti-aliasing filter). In addition, some Raw converters use a demosaicing algorithm that's not the best at reducing the negative side-effects of aliasing. For example, Raw Therapee's Amaze algorithm, and Capture One's algorithms are better at extracting high resolution detail, while at the same time reducing the artifacts as much as possible (it will never be fully avoidable).
Because the TS-E lens has higher contrast near the resolution limits, the aliasing will be more visible, just like the detail is more visible. The effect will be strongest with your lens at apertures around f/4 to f/5.6, and narrower apertures will produce a more even sharpness from center to corner, but also be a bit lower contrast and less susceptible to aliasing due to diffraction blur.
As the resolution of sensors gets higher (smaller sensel pitch), the aliasing will reduce, but it would probably take a bit less than 1 micron sensel pitch to practically get rid of the aliasing, so we're not there yet, and these artifacts are unavoidable without reducing image contrast, especially with good lenses and good technique (tripod etc.).
Cheers,
Bart