Why do time entries appear as non-billable altough the engagement, project and task are billable?

This behavior occurs because a change of the task from Non-Billable to Billable has been done with an effective date, and the time entries have been submitted before the effective date of Billable status change.

The time entries can be desubmitted and resubmitted after the effective date billable status.

Alternatively, change the task from billable to non-billable and back without specifying an effective date. In that case, all time records will be affected.