It happens if you have 8-bits set in the ACR settings down at the bottom but not if you have ProPhoto 16-bits set. It may have to do with color-space conversion from internal ACR workspace to whatever is converted to on the way out of ACR.
A slightly shorter way to accomplish opening a TIF in Camera RAW is to use PS: File / Open As... and choose Camera RAW when you reopen the TIF.
Here are a couple threads about the subject a few months back:
Sorry I was not able to find those in my searching.
The reading on dithering and all was interesting. However the fact that this is suddenly new to ACR after it has worked differently for the last how many years says it is a bug. I would prefer not to add another two steps to my workflow by processing this or that way to get around it. Has there been no official recognition of the problem?
BTW, thanks for linking to those.
I would prefer not to add another two steps to my workflow by processing this or that way to get around it.
I agree that the dithering regardless of settings is a bug - the Camera Raw color transform should respect the Color Settings choice you have made in the Color Settings preferences panel of Photoshop, and it does not:
Are you opening your images into Photoshop?
If you set the converter to output 16 bit data, then just work in Photoshop with the 16 bit data, then Save As or Save For Web & Devices there aren't really extra steps... And all your processing will be in the more accurate 16 bits/channel color depth, which yields better quality images overall.
I would also suggest converting to the ProPhoto RGB color space to avoid channel clipping due to gamut issues.
It's pretty clear that because of quirks in Camera Raw, extra steps may be warranted to get the best possible image quality.
P.S., There was another thread where this was discussed extensively, mid to late last year.
The second discussion link I posted, earlier, says this happens in ACR 6.2, so it is not new with the latest version. The chart also suggests that you can set ProPhotoRGB 16-bits and avoid the issue.