subtasks should 'stick' as soon as you enter them.
If you add a subtask, and hit "save and close" the subtask doesn't get saved. You have to hit 'enter' in order for your subtasks to be remembered. Hitting 'save and close' or even closing the task, should automatically save your subtask
Thank you for sharing your feedback. Right now we have other important things in the list however, I have shared this with the founder of the WeekPlan to consider it to add in the backlog.