We are trying to use the button for filter pane as well. The logic is [EnrollmentDate] >= 1/10/2019 and <= 2/1/2019, and [StudentID] is blank . What is the syntax for the field defined expression (why concat is needed) ? Is there any documentation available?
Had the same issue, couldn't find meaningful documentation, even it is an official Qlik extension.
In the end, did not use the button for this purpose anymore. The reason is, even it is working fine, it is 10 times slower then the filter pane. The processing of such expressions may work differently then in a filter pane. With the current Qlik version I recommend against it.