I don't have a save after, because my wife insisted that we do this again ... and again ... after each bug and result, but I have the save game immediately before.
To reproduce the bug:
Unchain Sebille and keep her stealthed outside of sight from the Shadow Prince.
Start the dialog with between the red and the shadow prince. Oddly enough you get Sebille's character icons in the following dialog.
Afterwards start Sebilles Dialog with the Shadow Prince.
Do the Combat Encounter
Talk to the Red Prince about what just happend.
Talk with the Red Prince again to the Shadow Prince, this time his Ghost.
