My headcanon is that the Doctor regenerates into a notable face from his past, someone he admired when he knew them or someone who was brave, or saved him or his companions. But because he’s so old, he doesn’t recognise himself after the regeneration. So when the Doctor regenerates as Capaldi, it’s because his mind retained information about Lucius, and projected that face onto his new regeneration.
Maybe it’s like how you only dream about faces you’ve seen ?