The contextInvariant for the displayCategory extension is currently set to be "type!='display'".

This extension is intended specifically for use with display items. For example, note the detailed description of the extension's value[x] property:

"Identifies the purpose for a "display" item."

Recommend changing the contextInvariant to be corrected to be "type='display'" instead.

Briefly discussed in fhirchat: https://chat.fhir.org/#narrow/stream/179255-questionnaire/topic/questionnaire-displayCategory.20contextInvariant