That data source expression pulls the first task in your task list - across all users and all dates. When an element is dropped on a DropArea, and it can be any element really, we could even drop an elephant (joking) - then your workflow will run for that first task, that task’s data will be changed - regardless of what task you’re actually dropping on it.
So, you can only change 1 task and the external repeating group has to include that exact same task.

But that’s not what you want, what you actually want is to modify any task that task you are dragging from your external repeating group positioned on the top right side of the page.
As said, this is not possible with this plugin.