All subtasks should be assigned by default to the same project(s) as the parent task. I understand that there may be occasions when someone would want it to be assigned to a different project but I would venture to say this is very much the exception and not the rule.
I encountered this when searching to try to understand why my subtasks were not appearing in my calendar view. The “workaround” was to manually go in and assign each subtask to the project (even though, again, they are simply a part of the larger task that already has a project assigned).
The very nature of the task-subtask relationship makes the necessity of a workaround a real headscratcher for me.