Filtering a lookup on checkbox values yields unexpected behavior

Filtering a lookup on checkbox values yields unexpected behavior

In my Personnel form I have a checkbox field named "Options" that has 2 checkboxes: "Foreman" and "Unavailable". Foreman marks an employee as a foreman. Unavailable means that employee is not available to be assigned to a job.

The Job Schedule form has a lookup field that sets the foreman for a job. It looks into the Personnel form and has a filter set to only show Foreman who are not Unavailable. This is done by setting the filter to "Options" Contains "Foreman and "Options" does not contain "Unavailable". 

Unfortunately "Unavailable", the second checkbox, seems to be ignored.

Employee 1 is a Foreman who is Unavailable
Employee 2 is a Foreman who is not Unavailable
Employee 3 is neither a Foreman or Unavailable

So the lookup shows employees 1 and 2 in the list. Removing the "Unavailable" clause from the filter keeps it at employees 1 and 2.

What could I be missing?