Numerous fixes went into color display on MacOS for Acrobat DC as well as in Output Preview in general. That having been said, I don't recall any changes that would have resulted in the phenomena you are seeing.
I assume you were clicking on the “black” circle to get you sample color values? And what do you believe should be the correct values? CMYK=(0,0,0,1) or CMYK=(0.8,0.64,0.67,0.75)?
If you can post a copy of this file, we can examine it to see what is going on.
- Dov