Actually, this is a bug that need to be fixed in the next release

When a project is deactivated, project task also should be deactivated automatically.
For a deactivated project in Task Tab, an error message “You can’t edit this project because it is inactive” should be displayed and project task should be deactivated so we could not add new project task.

But somehow, it works intermittent. Mostly project task is still active when we deactivate the project.