And for the others.
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The Readers for supported file types (off, om, ply, stl, obj) all use the add_face function of the templated binary importer class which adds non manifold faces as separate faces, and marks them as fixed nonmanifold face.
So as of today, all readers handle complex faces using the base importer. Did you mean to open this issue in OpenFlipper instead, and want me to check OpenFlipper readers?