For anyone still experiencing this issue - navigate to the project page where you should normally see the “sections”. Navigate to the fields at the top of the project. Where it says “Subtask of”, delete that field.
Try again and see if that works.
Experienced the same issue, until I deleted that field. For some reason, it recognizes it as a type of custom field for tags. My subtasks were displaying under my parent tasks, while the name of the parent task merely displayed in this field - without it being added to the parent task.
Try it, hope it will alos solve your issue.