Can't set issue start/due date when field values are calculated from subtasks and subtasks have no such fields.
Steps to reproduce:
1. Enable calculation of issue start/due date values based on subtasks in Redmine settings
2. Create a parent issue that has start/due date fields enabled
3. Add a subtask with a tracker that has start/due date fields disabled
Start/due date values can be set on parent issue
Start/due dates for parent issue can't be set neither in parent, nor in subtask.