Filters yielding too few results when PET field set to IS NULL

My team and I realized something disturbing, that if you create a filter and the first filter item is a PET table field set to "is null" then the results you get back will be limited to records with rows in that PET table.

This is quite unexpected. Our previous understanding was that we should be receiving back ALL the records from the main List, minus any records where the PET table field has a non-null value.

What is crazy is that if you start with a condition that references the main List (e.g. CUSTOMER_ID_ is not null) then you do receive the full results.