Keep in mind that the Dialogue Manager GameObject will survive scene changes and replace any Dialogue Manager that's in newly-loaded scenes.
If your player GameObject is in the gameplay scene, don't assign any inspector references between the player GameObject and the Dialogue Manager GameObject that's in the gameplay scene. That Dialogue Manager will be gone when you arrive from the title scene, since the title scene's Dialogue Manager will supplant it.
Also, if you have scripts or configurations on the Dialogue Manager in the gameplay scene that aren't in the Dialogue Manager in the title scene, they'll be gone when you arrive from the title scene, since the title scene's Manager will be present. So add those scripts to the title scene's Dialogue Manager.