Overview
As of today, when a user unfold a parent item (epic), children (user stories) that are in the backlog of a release but not yet planned into a sprint, they are not displayed at all (because there is no timeframe).
However if a story is in two sprints, the story appears as a child, without the progress bar (and with a warning).
As it's it's important to envision the whole content of an epic, all children should be displayed, even if they don't have a timeframe.