I agree that you might think the empty matching field would generate a record in the *import into* table with an empty id field, but that does not occur.
The key may be in this note in the help file:
If all specified match fields in a record are empty, that record will not be updated.
This statement is referring to the receiving record, but it makes sense to think of it in terms of the *sending* record as well.
My guess is because the matching value in the 6th incoming record is null, that is seen as a non-record. There is nothing there, therefore, there is nothing to import.
If you have a record in the receiving table with a null id, it will not receive the incoming data from the sending table with the null id record. Nothing matches nothing, or in this case, nothing does not match anything.
Yep I missed that in the help files.
Thanks for your helpful reply.