Dotted stroke when applying the Rotation Tool

How solid or dotted a brush stroke appears is controlled by the Spacing setting in the Brush Tip Shape section of the Brush settings panel. Reducing this value causes each brush tip shape to overlap the previous one more readily. I'm guessing the angle settings are interacting with the rotation in an unexpected way here. What is the Angle value? Maybe aligning the brush angle to the canvas angle is making the spacing more apparent than usual.

Your document window is zoomed in to 396% and you are using a 2 pixel brush preset.  What you are seeing is probably a display artifact caused by the angle, zoom ratio, and anti-aliasing.  What do you see if you hit Escape to cancle the roation, and set the display zoom ratio to 100%.  That's the only thiong that counts or matters.