The flattening works based on the vertex weights as far as I know. If ac3d supports that then check these weights on your taxiway mesh.
The flattening works based on the vertex weights as far as I know
That's what I thought in the past as well. But meanwhile I have learnd, the weights are exclusively responsable for the opacity of the ground material.
Moxeetwo, could you maybe show the ground model how it looks in your 3D program? It seems to me a bit like it would end in the area of the hold short.