What we're trying to highlight, is that that finish is down to cutter/work piece deflection. The fact it's only noticeable in certain materials, doesn't make it a problem with the material.
If you zoom in on that last pic, the problem is still there, it's just not as obvious.

Different cutters and materials will create different cutting forces and resonances, so what can be barely noticeable with one material and cutter combination, can become glaringly obvious by just changing either the material or cutter. And if you have any flex/backlash in the system, any issue can get amplified.