The copy and paste functionality confuses the task groupings for reordering. Steps to reproduce:

1. Create nested tasks:

A task
A nested task
Another nested task

1. Copy and paste one of the sub-tasks

A task
A nested task
A nested task
Another nested task

1. Try to reorder any of the subtasks and the entire set of tasks moves as one unit. I cannot reorder the tasks into this arrangement:

A task
A nested task
Another nested task
A nested task

The "A task" item and everything under it moves as a single unit whenever I press shift, click, and drag any of its subtasks.

Deleting any of the subtasks restores the expected functionality.