In the new CS6, the same Pantone is broken with C100 M65 Y28 K00 values, which is completely out of the range... Is there another way to fix it ? Is there any chance to go back and use a good, reliable, CMYK separation system with CS6 ?
Are you sure about the 28% yellow—I don't see that much yellow with any document CMYK profile.
The conversion from the Lab values depends on your document's CMYK profile and the Color Setting's Conversion Options. Europe ISO with Relative Colormetric and Black Point Compensation turned on returns 100|58|0|0, while US SWOP Perceptual with Black Point off returns 100|65|13|1. So as Peter mentions you have to have the correct profile in place for Lab conversions to work.