This one's been around thru several versions.
I don't know what brings it on but at times the dropper selects only white as if it can't see through the current layer.
The current example is a "normal" transparency layer with the drawing outlines while the body colours of the object are painted on the layer underneath.
"Sample:All visible layers" is selected, but isn't working as instructed.
On the current image I've found a workaround, turning the "normal" layer to darken/multiply/gel makes the empty spaces transparent to the dropper, though they should already be so.
Also selecting "Sample: selected layer" works nicely, ignoring the layers above it.
I can't be sure but I suspect I've had trouble reading thru the transparent spaces on gel layers before also, though it appears not to be the case in my current image.
This bug isn't always there, I don't know what brings it on.