Well, I tried my best... I can't really think any more simple example to make than the single polygon one.

Just for the record and as others noted above, the 4 vertex normals in the single polygon example are not all the same, there are different signs between the z values:

vn 0.00000000 -0.70710678 -0.70710679
vn 0.00000000 -0.70710679 0.70710678
vn 0.00000000 -0.70710679 0.70710678
vn 0.00000000 -0.70710678 -0.70710679


You're just going to keep on getting bug reports from users of many different CAD programs until this issue is resolved though. It would really save so much effort in the long run to have it fixed up sooner rather than later.

- Michael