I’ve read fanfics and also seen some posts about the way Jonah reacts upon hearing his original name. That it feels nostalgic, that he enjoys hearing that he likes to be called Jonah again…

Let me raise you completely opposite theory - Jonah has no reaction at all. If someone calls him Jonah it takes him very long time to even notice that they are talking to him.

It just hasn’t been his name for so long it doesn’t feel like his anymore. He’s used to hearing it in passing in his Institute - Institute founded by Jonah Magnus - that there’s no reason to have any reaction to it; it’s only natural desensitize to the name.

He is more likely to react on being called James. It’s not so long since he was James. But it’s very long since he was Jonah…