Scripts configured for execution before oder after backup are executed before backing up every single datasource configured in that time plan/schedule.
There should be an option to execute the scripts only once befor starting the backup for the first datasource and after finishing the last of the configured datasources. To achieve that behaviour is not possible right now.
Sure: If a script is only needed to be executed before one specific datasource, a new time plan/schedule can be set up. But having a script executed after the backup of the last of a set of datasources has finished cannot be implemented that way.