I am getting the feeling that LeoCad itself can’t export properly.

It looks fine in 3DS Max because back-face culling is not enabled.

Enable it and admire the artifacts.

It seems I need to write my converter for those .dat files after all.

The only thing you can do is manually fix the faces in 3DS Max (not very practical), write your own .dat converter (only slightly more practical), or disable culling (performance warning).

L. Spiro