Hi erik
Sorry, but this isn't an error.
As you may know, WordPress uses MySQL to store the site's information.
ATUM Export works exporting this info by using SQL sentences. These sentences include different tables related to each other.
Said this,
NULL cannot be treated as a normal value because it is not a normal value. This means that when you use the 'NOT IN' clause, you are looking for values that are not in..., and it is not a value.
And, on the other hand, Product visibility is a taxonomy. The taxonomy terms are stored in the database in tables apart. No record is inserted in the taxonomy tables if the product has no related terms. So no record will be exported when using the IS NULL clause.
At this moment, the only way to include the NULL values is by exporting without applying filters to the product's visibility (or any taxonomy).
We're working on a workaround for this. The IS NULL clause will include the non-existing terms for taxonomies in a future.
Best Regards,
José Andrés