The morph line is probably the easiest. Another solution is to split the slab at that point, and you will get the edge line in 2D elevations and 3D views, but will disappear in sections. However, you will have the split line in 2D plan views, unless you turn off the slab layer. I agree that this is something that should be fixed.
Richard
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Richard Morrison, Architect-Interior Designer
AC26 (since AC6.0), Win10