When pyplot.boxplot() receives a list of lists one of that is zero-IQR as input X, box and whisker plots of subsequent columns of X will have (min, max) whisker regardless of the value of whis.

For example,

X = [data, [0.5], data]     
plt.boxplot(X) # whis = 1.5, flier points should be plotted for the first and third columns of X.

the script like above will draw the following figure.

I suspect this commit.