In Service Portal when a reference field is configured to display more columns, it is displaying the columns down the select box, rather than across. When inspecting the element you can see that it is missing display: inline-block; which appears on patches later than Istanbul Patch 8.

For example:
width: 33.333333333333336%;display: inline-block;

Percentage may differ depending on how many fields are being displayed.

1. Create a catalog item with a reference field targeting the Incident table.
2. Add the following attribute for that variable:
   ref_auto_completer=AJAXTableCompleter,ref_ac_columns=number;short_description;sys_created_on
3. Navigate to the new catalog item on Service Portal.
4. Click on the reference field select box.
   
   Columns defined are displayed down the select box for each value returned

Workaround CSS can be configured on the page or widget. 

.select2-result-cell { 
width: 33.333333333333336%;display: inline-block; 
}