I had the same issue. After exporting a Maintenance Plan from SQL Server Management Studio and importing it into a new Integration Services project using SSDT I noticed that for some strange reason the system generated Back Up Database Task wouldn't allow me to set an expression for the "SelectedDatabases" property. It simply wasn't in the list. The same wasn't true of the system generated Maintenance Cleanup Task.

Low and behold, dragging a new Back Up Database task onto the Control Flow 'fixes' the issue.