Round dots get square at 50% grayscale while converting to bitmap screening

I have a grayscale image of 5x5 cm at 2540 dpi. I convert it to bitmap through Image > Mode > Bitmap. First I choose the settings Output 2540 lpi and Halftone Screen... In the second window I use the settings 90 lpi, 7.5 degrees and Round shape. In the resulting file I notice that around the 50% dots are getting square instead of round. How to change this to fully round dots?