Since your chart is shown using a stereographic projection, the stipple marks closest to the pole only appear like a line. This is because the distance between longitudinal coordinates decreases as you move poleward, and so the stipples are essentially being plotted on top of one another.
One potential way to resolve this would be to decrease the 'density' name-value pair and/or the marker size for your chart:
stipple(x, y, mask, 'density', 500, 'markersize', 2)
Alternatively, if you don't want to apply those changes to the entire figure, you can try making an index for all the stipple marks above 70 degrees N latitude and then plot those values separately from the stipple marks at lower latitudes, but with a different density and marker size.
stipple(x_below70, y_below70, mask_below70, 'density', 1000, 'markersize',3)
stipple(x_above70, y_above70, mask_above70, 'density', 500, 'markersize',2)
If neither of these options work, consider attaching a .mat file containing your data (or a fake dataset with a similar issue) and then either myself or another one of the volunteers can try to figure things out on your behalf.
