I have exactly the same problem and I am sure it is a software problem, not hardware.
I just got an Officejet 7612, I run on MAC OS 10.10.5
The problem is, the scanner's software does a compression of the file even before saving it in an uncompressed format. The result is, we can see BAD JPG compression artifacts on uncompressed TIFF files. It doesn't matter if you scan at 200 or 1200DPI.
The data dosen't go directly from the scanner to the final file without the unnecessary middlewares for smoothing and compressions, there is already a lost before saving.
The "uncompressed format" is a lie, this is a false statement in your settings.
Plus, the "High quality JPG compression setting" is far from High quality standard, at best it is equivalent to a 50% quality compression (in photoshop for instance, I just did the test). We shouldn't see JPG artifacts in a HIGH quality JPG compression, at least not that much.