I just tried it with "Bilinear" and "Bicubic", and the result looks much better.
So, there seem to be two issues:
1. "Lanczos 3 (non-separable)" creates the double outline artefact.
2. Resampling does something, even though the original image was 640x640 and it was also exported to 640x640. Does that not make resampling obsolete?
Cheers,
Frank
P.S.: I guess I'll just use Bicubic then, since that sampling method is superior to bilinear. However, maybe it's still worth looking into, I guess. Either there's a bug to fix in the sampling code, or the help could be amended with a notice that Lanczos is maybe more suitable for photos, and less for line drawings?