Edit: I just wanted to add for clarity that I know you can add a tag to a subtask by going into the subtask first. What follows is a description of how the behavior should work.
Here is what I would expect the behavior to be when you are managing subtasks:
• In the detail view for a task that has subtasks
• Select a subtask
• Pressing Tab-T allows you to tag the subtask
What happens now is it tags the main task, not the selected subtask.
Here I have selected a subtask:
Tab-T brings up the Tag field for the main task: