Problem Definition
As per this article, the description of the function mentions that the function applies to issues in the 'Done' column, which should be the last column. In reality, the affecting factor would be the 'Status Category'. Regardless of the Column, the issues affected by the function would be dependent on the 'Status Category' set.
This causes some confusion in understanding the expected behavior. Also, there are some use cases where the Status with 'Done' Status Category would not be in the last Column, yet the issues would be hidden.
Suggested Solution
The 'Hide completed issues older than' function should only apply to the last 'Column'. Or have an additional configuration to set which Status would actually be affected by the 'Hide completed issues older than' function.
Workaround
If the Status is in any Columns other than the last column, set the Status Category to 'In Progress'.
+1 this is very misleading.