this is typically how Moodle activities work - when a user is un-enrolled from the course, the information associated with their grade/assignment submissions (and attendance) are removed from view in the reports - If you would like to retain this information in the course reports you can "suspend" a users enrolment rather than remove it, which prevents them from accessing the course, but keeps the data in the reports visible to teachers.
The data for un-enrolled users is still retained within the attendance tables, so it's possible we could improve the attendance reporting to include un-enrolled users, but that would likely require funding for my time to improve the reporting.
The longer term aim is to replace some of the reports within attendance with the new Reportbuilder API that is available in moodle 4.0 and higher though - so I'd prefer to see that work completed first before we added further functions to the reports.
The data for un-enrolled users is still retained within the attendance tables, so it's possible we could improve the attendance reporting to include un-enrolled users, but that would likely require funding for my time to improve the reporting.
The longer term aim is to replace some of the reports within attendance with the new Reportbuilder API that is available in moodle 4.0 and higher though - so I'd prefer to see that work completed first before we added further functions to the reports.