Subtitles are OFF by default, so I'm not sure why you'd want / need to have the intro media's end action the way you do. Maybe by having the intro media set to specifically turn "off" the subtitle, your DVD player is interpreting it as an instruction to turn it on (since you can't turn off something that's already off -- makes sense?).

Try putting your intro media's "Set Subtitle Stream" setting back to "No change."

Rob