The sys_tag field added as secondary field on "Knowledge Result" widget in doesn't display on knowledge portal . While other reference field like kb_knowledge_base get displayed when added as secondary field in the widget.

Able to reproduce the issue on London, Madrid and New York instances

1. Log in to a Madrid instance
2. Navigate to the knowledge portal and search for a keyword
3. Command + right-click on Knowledge Result widget and open Instance in Page editor
4. Add Secondary Fields like "article_type,sys_tags,active,kb_knowledge_base" to display on each knowledge result
5. Observe that "knowledge_fields": Article type, Active, knowledge base get displayed for knowledge result except for sys_tag field
   
   Expected behavior: Sys_tag field should also get displayed as knowledge base field and other fields
   Actual behavior: Sys_tag field doesn't get displayed as other fields on knowledge results.

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