Adobe Illustrator's Outer Glow on large border not generating correctly; Strange horizontal lines of incorrect rendering

[alexw2801243]

I am currently attempting to make a map and would like to apply the outer glow effect to the continent to give a slightly lighter blue coastline.

I select the (closed) outline of the island and choose the Outer Glow effect (with a Mode such as Normal, Screen or Overlay), choose 95% opaque, and a blur of 50 px.

The below is a screenshot of part of the outcome.

As you can see there is a horizontal line in which the outer glow is not being applied correctly. I would like to know how to fix this.

I notice that this horizontal line applies with a consistent vertical spacing and happens on both sides of the continent in a horizontal line. My map is very large in terms of pixels, and I wonder if this is what causes the problem. If it is important, this operation takes around 15 seconds to render.

Thanks, Alex

Edit: Changing the blur pixels amount changes the position of the horizontal lines. Changing the opacity and/or mode does not.

[Ray Yorkshire]

How does it look with

View > Prievew on CPU ?

[alexw2801243]

Thanks for the reply. Sadly it still appears so here also.