Activity disappears from 'Overview of students' report after student has completed it if the activity is on a Onetopic course format child tab and the parent tab has access restriction by activity completion.
The plugin page https://moodle.org/plugins/block_completion_progress links Bug tracker to: https://tracker.moodle.org/issues/?jql=project%20%3D%20CONTRIB%20AND%20component%20%3D%20%22Block%3A%20Completion%20Progress%22 so there is an issue created here also: https://moodle.atlassian.net/browse/MDL-86689
Affects versions
4.5.6