Custom roadmap does not consider the Parking lot start/end date fields

The parking lot has built-in standard start/end date fields. However, when using a custom roadmap, these date fields are not considered.

Instead the parking lot bar extends the full width and is faded at each side.

While the parking lot doesn't have a release date since it's unplanned work, it's unclear why the parking lot start/end dates are not respected by the custom roadmap.