There is no such thing as true curves in Archicad.
All curves are made up of multiple straight lines.
So the mesh you are creating on each side of the curve may have different faceting on the adjoining curves, resulting in the 'gaps' causing your 'fencing'.
The best thing you can try s to ensure the start and end points of the adjoining curves are the same.
That way they should facet the same and you won't get the 'fencing' problem.
No guarantee though as you can't always control the faceting.
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