Ideas

The product import is not able to remove existing product relations, only add new ones. When exporting a product with the relations fields, removing a related product and the reimporting nothing is updated. During the import the products are counted as updated, but no relations are updated.

Same for both directional and bidirectional relations. I understand that it could be an issue to update bidirectional relations if the related product is also in the same export file (and still contains the relationship information) but this is not the case here.