Add new subtask - parent issue id is lost in some cases
When the tracker of an issue has the
parent_issue_id field disabled and one tries to add a subtask, it may happen that the id of the parent gets lost if the tracker that gets preselected in the new issue form is the same or also has the
parent_issue_id field disabled.
The attached patch which we developed at Planio fixes this by always passing the id of a tracker with the
parent_issue_id field enabled (is possible at all) through the 'Add subtask' link.