I was shocked when I clicked “Remove as Subtask” from the action menu of a subtask, as it was completely gone from the project. I panicked for few seconds before I found the “Add to projects” option and added it back to the right project and section.
For some strange reason, subtasks of a task in a project (and section) are not linked to any project? So removing one of them as subtask just pushes it into the abyss (I mean, the archive)?
I think Asana should automatically link it to the same project (and section) of the parent task, or warn about it (giving a choice), or open the Project linking drop-down box (with current project/section on top for quick selection). The behavior could be dependent on whether the subtask already has a project/section linked to it or not.
You can see here, another person encountered this same issue (and then lost access to the task as he navigated away): How do I get my subtasks back once I use the option 'remove as subtask'
So this would be good to prevent subtasks from disappearing from the project.