Gantt changes project start date automatically.

The gantt automatically changes the project start date to default to the first day of the gantt chart if their are no children tasks.

How can I stop this default behavior?

Unfortunately there is no built-in solution for it.
Tasks with project type ignore start_date, end_date and depend on child tasks. I.e. by default they should have at least one child to display correctly.
The option of creating a project without children is not provided.
Possibly you can to redefine this behaviour by changing the source code.

Thanks for the input!