As reported here: https://www.reddit.com/r/AdobeIllustrator/comments/1p6ecuj/behavior_of_eyedropper_tool_changed_recently/

In previous versions (can’t say for sure now when it stopped) we were able to aim with the Eyedropper better, when Smart Guides were enabled. In CC 2017 it works. Was quite helpful to pick color from a stroke, highlighting the object that is going to be sampled. Cool, right?

But now, I suppose with the Photoshop-style sampling rings glued on, Illustrator forgot it’s a vector program. The sampling ring should not disable Smart guides!

I somewhat get why pressing Shift might disable it, because Shift enables the 'sample by pixel' mode, and picking a color from a smart guide is not something we would need (actually, jus ignore the color of these, don’t hide)... but why disable Smart Guides for the normal mode? Please review and fix! The tool doesn’t hint what exactly it would pick, a stroke, a fill, a raster color... Sampling ring is not a solution, especially if you work in CMYK.