What I've suggested is basically a replacement of the standard bevel, but using the bevel to get the shape to draw.

As for the issue more generally - artefacts are likely to occur with quite a few of the effects if the polygon extends a long way outside the edge of the rendered area, since some of them rely on the render engine being able to 'see' the whole polygon. A small distance outside the map is ok, but once you get beyond a certain distance away it all goes a bit wrong.

There is a way to fake a smooth-topped bevel.

If you have Darklands overland it's set up using a white glow and a black shadow on the WATER sheet of that style, though for some reason I had to increase the width of the second glow from 2 to 5 units when I reopened my example map.

The bevel is 'cast' onto the land by the water, which is on top of the land.

The effects that create the illusion are the last two in the list.

However, I appreciate this may not be practical in a battlemap.

@JulianDracos Sorry - I meant your FCW of the map you are having problems with.

30 minute shout :)

Should the top end of a carnivorous root be blunt and black in section like a pillar, fade out, or something else?