There is a task in 'Done' lane that is now gone. I tried to refresh the window and also did a logout/login but the task just disappeared.
Arup Maity commented
There is a setting for view filter for the tasks in the done lane, just to manage the loading for those project that gets used over years, if the date filter can remove the display of the tasks that were done before the applied filter date. It does not mean they are gone, they are still available for viewing if you change the filter date