1)You can not share subtasks individually.
You can only share parent task with all their subtasks.
Will you change that in the future?
2)Testing new sharing (Joint task) i've noticed that when a collaborator remove sharing on a parent task, parent owner's task changes to unshared but not their subtasks, wich remain shared. I think that this is a bug.
Thank you for your new improvement!
That's what we will looking for.