If you have a clock connected that always display same thing (Appear number: 1 100000 1, title: {text:" "} subtitle: {text:"Write anything there, in my world I'm using a scoreboard-display"}), and if you reset the world, then go to a world, where you can active a /title @SELECTOR title... The subtitle will transport
What do you mean by "reset the world"? Are you exiting and saving? recreating the world?
It's intended that subtitles persist through multiple uses of the same title, within the same world. See @unknown's example.