Strange pdf render in Safari

[groupoid]

Symptoms: whenever I open a pdf file in Safari, it first displays normally for a short time, but then makes a shift as on second attached picture.

1) Does only happen to pdfs that have non-vector graphics (such as scanned books).

2) Does not happen to pdfs generated from TeX or word processors.

3) Same thing happens in Preview, but only in thumbnails, not in the actual file.

4) Verified using gfxcardstatus: happens both on integrated and discrete GPU.

All latest updates are installed. Configuration of my macbook attached.

Thanks!