If a learner enrolls in multiple sessions before any session is marked as attended, they get marked InProgress when they complete one or more sessions if it's not all of sessions they are enrolled in. When they complete all sessions they are enrolled in, they get marked as Complete for the course and the course completion date is the date of the last session completed.
If instead, they sign up for an additional session after they have at least one session marked as attended, their status stays as "Complete" and the course completion date doesn't update when they get the additional session marked attended. Oddly, if they then sign up for another session, the completion date updates to the second completed session date even though the course status does not change.