First reported by @DavidLemayian. Comparison of the same PDF, one whose thumbnail was generated in February 2015, and one generated today:

But it's not a straight regression in image quality. The old downsampling looks better at thumbnail/small sizes, while the new looks better at normal/large. Click each ❌ or ✅ to view the image.
| Size |
Old |
New |
| Thumbnail |
✅ |
❌ |
| Small |
✅ |
❌ |
| Normal |
❌ |
✅ |
| Large |
❌ |
✅ |