Solution
Add the same Formula type to the Available Filter tab on your Saved Search as on your Criteria tab.

Note:
- You can use two different Formula Types as Available Filters, for example Formula (Text) and Formula (Date).
- If you add two Criteria with the same Formula Type, the Available Filter formula will apply to both
- You cannot add the same Formula Type as an Available Filter twice.
- You cannot use Formula (Numeric) as Available Filter.