If you want to create a polygon from 3 vertices you have to connect them. You can make 1,2,3 or 3,2,1. The direction in the file must match the direction in the 3D app. Unreal has chosen a different way than all the others. You may also know this from the Z axis instead of the Y axis for up and down. I try to compensate this in the plugin but with some files you have to select this yourself otherwise they are mirrored.
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