Redmine

I found following problem when set delay for one issue.
If the start date of the following issue is Saturday, the system will move the start date to next Monday automatically.
However, if the start date of the following issue is others (across the weekend), the system will not take non-working days into consideration.
Please check the following example.
Is it a bug or my setting issue?

• Non-working in my system is Saturday and Sunday.
• Issue A: 2013/09/02(Mon.)~2013/09/03(Tue.)
• Set issue B follows issue A with N-days delay
  • N=1, Issue B start date: 2013/09/05(Thu.) => Correct
  • N=3, Issue B start date: 2013/09/09(Mon.) => Correct
  • N=4, Issue B start date: 2013/09/09(Mon.) => Incorrect, SHOULD be 2013/09/10
  • N=6, Issue B start date: 2013/09/10(Tue.) => Incorrect, SHOULD be 2013/09/12

System information:

Environment:
  Redmine version                2.3.2.stable
  Ruby version                   1.9.3-p448 (2013-06-27) [x86_64-linux]
  Rails version                  3.2.13
  Environment                    production
  Database adapter               Mysql2
Redmine plugins:
  clipboard_image_paste          1.7
  redmine_projects_accordion     0.1.0
  redmine_tab                    0.4.0
  redmine_theme_changer          0.1.0
  redmine_watcher_groups         0.0.2git
  redmine_work_time              0.2.8
  timelog_timer                  2.0.0