The description of next-q should clarify with regard to the server's response:

1) That more than one new question might be returned in the questionnaire sent back by the server.

2) That previously sent questions that were unanswered might not be sent back by the server.

See https://chat.fhir.org/#narrow/stream/179255-questionnaire/topic/next-q.20returning.20a.20group.3F