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

i attempting make map , apply outer glow effect continent give lighter blue coastline.

 

i select (closed) outline of island , choose outer glow effect (with mode such normal, screen or overlay), choose 95% opaque, , blur of 50 px.

 

the below screenshot of part of outcome.

as can see there horizontal line in outer glow not being applied correctly. know how fix this.

 

i notice horizontal line applies consistent vertical spacing , happens on both sides of continent in horizontal line. map large in terms of pixels, , wonder if causes problem. if important, operation takes around 15 seconds render.

 

thanks, alex

 

edit: changing blur pixels amount changes position of horizontal lines. changing opacity and/or mode not.

how with

view > prievew on cpu ?

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