It sounds like you're using the "CurrentAssignment.Group" relationship to get the team name. The CurrentAssignment gets removed once the assignments been completed (ie. when your Change has closed). Instead you can use the "LatestAssignment" relationship which is identical to CurrentAssignment except it doesn't get removed.
If you change the query to display 'Latest Assignment.Group.Title' instead of 'Current Assignment.Group.Title' then you should always see the Group name in the results assuming that the last assignment had a Group specified. The current assignment of an object is 'Completed' when the object moves to a status in the process that is an 'End' type, resulting in the object no longer being assigned to anyone.