empty boolean import as 0 when not in mapping

I am importing products, just basic valeus (sku, description, picture, ... ) not the attributes yet. So for this importer i have only a mapping with the basic values, not with the attributes in it. Nevertheless, the boolean field attributes are populated with value 0 (also in database). There is no default value in the field settings. Upon creation of the boolean field, I do not change a thing to it. I leave the fields blank so 0 for negative and 1 for positive.

If I import basic values + attributes and let the attributes empty, then this doesn't happen. But this way of working is not feasible, because then I have to specify all the boolean fields in an import even when they have nothing to do with the kind of product that i am importing.

someone has ideas what this could be? I dont find related info on the net.