Este é um comportamento documentado. O processo de primeiro plano é concluído depois de bifurcar o processo de segundo plano. Na página do manual:
-b, --background
Typically used with programs that don't detach on their own. This option will force start-stop-daemon to fork before starting the process, and force
it into the background. Warning: start-stop-daemon cannot check the exit status if the process fails to execute for any reason. This is a last
resort, and is only meant for programs that either make no sense forking on their own, or where it's not feasible to add the code for them to do this
themselves.