You_Too said:

EDIT: I opened the blu-ray in avspmod using a script and there it's smooth. So it's MPC's fault.

It's not necessarily MPC, it might have something to do with the renderer or maybe even your video card driver. Is it the old MPC you're using or MPC-HC?

An example of the issue, here's a conversion using bicubic upsampling (Avisynth's default):

MPEG2Source("VTS_01_1.d2v", cpu=0)
Crop(0, 102, -0, -102)
ConvertToRGB(chromaresample="bicubic")

And here is the same frame with point (i.e. simple doubling of the chroma) upsampling:

MPEG2Source("VTS_01_1.d2v", cpu=0)
Crop(0, 102, -0, -102)
ConvertToRGB(chromaresample="point")