One character's development which is not working

I loved the 5 hours and a half of Nymphomaniac, and I'm impressed how Lars Von Trier managed to make me look at so much "lust" (to take his own word) without feeling uneased or uncomfortable.
Ok, apart from one scene: I just couldn't keep my eyes open for the abortion scene. Too much for me. Not the actual idea of self abortion, but the body mutilation is not about sex anymore. It's gore.

Anyway, I think all the characters are deeper than you could ever expect and that's extremely satisfying. It all makes sene, their actions, their developement, predictable or not, or their lack of developement.

All except one.

Jerôme.

Jerôme is played by Shiah Leboeuf, with both "Young Joe" and "Adult Joe", and, wit a bit of effort, it works.
So why would you replace him with another actor by the end of the movie, and give him this new violent disturbing image.

It is not the same Jerome anymore, literally and figuratively, it's someone else, and it is impossible to believe this is Shiah Leboeuf developement.
He could not become this Jerome.
Both actors have very different ways to act.

I built empathy for Jerome, he's not perfect but he turns out a good man trying hard to make Joe happy.

Yes, she tried to kill him, but this isn't Jerome's eyes I saw in this unknown actor *beep* an 18 yo in the alley in front of his half unconcious ex partner.

Why not keep using Shiah Leboeuf and see how this scene would have turned out to be?

That's a shame, because all the other characters are perfect. And it's hard to forgive, because Jerome is a major character.