For this issue, the color picker has to be set to "Current & Below", or "Current & Below, No Adjustments."
The problem is if you try and color pick your current layer inside a group that has any kind of layer clipped to it, it will ignore your current layer and only pick underneath it. In some areas, it sometimes picks the proper color, but typically it just ignores the current layer.
The only way to resolve this is to unclip the layer from the group, turning it off doesn't help. This has been verified as a bug in CC 2015.5 and CC 2017, all versions, by both me and a colleague on separate machines, both PC. I haven't tested any other CC versions, but it is NOT an issue in CS6.
Interestingly, on lower resolution files, it sometimes goes away, but it's persistent on higher res files. Also, in SOME documents, changing the sample to 3x3 rather than point, it will pick up the proper color in more areas, but is still glitchy. I wonder if the cache levels on the document might have something to do with it. Also, I removed any plugins and preferences from Photoshop to verify it was not an outside issue.