It appears that the backend is not processing the org.sagebionetworks.UNDEFINED_NULL_NOTSET value correctly for list-type columns when using that value in the facetValues query request field. It is incorrectly returning zero results.
Steps to replicate issue
Visit my demo project here - https://www.synapse.org/#!Synapse:syn22041545/tables/query
Under 'veggie list' click 'Not set'
Zero rows are returned instead of the two containing 'apple, pear' and 'strawberry'
One thing to note-
I noticed this issue arises when there are two or more list-type columns, with a single list type column there is no issue