OS 10.4.8 IDCS2
I am surprised I have never noticed this.
If you place a GS tiff in IDCS2, then select it with the direct select tool, and use the tint field in the swatch palette to screen it back to, say 30% of the default black swatch, then turn on overprint preview, the 30% screen will go away, and the separation palette will read 100% where there should be 30%.
It does not however print this way.
And this display anomoly does not occur with any of the other stock swatches.
As far as I can see there is ...