This article explains how to exclude some records properly in a custom way.
How to exclude records
When a custom query is needed to exclude some records from the result set, remember to add "OR [field] is NULL". This additional part is added after following OOB way to exclude records.
Filter out any record from any list view and inspect the filter created. The filter reads as "[field] is not <keyword> OR [field] is NULL"
Having only "[field] is not <keyword>" as a query will produce an incorrect result set. '[field] is NULL' records will also be excluded unnecessarily in this case.
This scenario is actually how the product works. MySQL is the reason to exclude records where [field] is NULL. According to MySQL definition, NULL means that the value is undefined. Therefore, those records where [field] is NULL are not comparable and won't be listed as a result.