I found out that you can't actually filter a subtask by it's parent issue fields in JQL without buying a plugin.
And I was very disappointed, the logic behind the parent-Child/Header-Detail is to group a bunch of data into an umbrella of a kind. Which means that all these child objects actually share the properties of the parent issue but differs in the detail, and that's why its frustrating not to be able to search through the parent issue's fields.
I will probably solve this using automation rule(by copying the needed fields onto the subtask), but that's just an extra work for such a basic function, I'm not willing to pay for a plugin for this purpose...
