When you don't specify the start date and the days in the config of the Gantt is should take the date range to show every single task. this, however, doesn't happen. instead, it takes the date range of the shortest task (I think).
I have made a small GitHub repo to simulate the bug https://github.com/WEBhicham/daypilot-date-bug
as you can see there. the parent task has a range from 01-01 till 01-31 and the child has a range from 01-10 till 01-15 but the Gantt only shows 01-10 - 01-15. (and apparently changes the dates of the parent)