Hey Anon! I’m doing okay, thank you. :) I hope you too!

I think there are several possibilities for your answer: firstly, in ch107, maybe Seb was on time to stop Lizzie because he’s the one who made sure Lizzie would show up in the first place. 

So I don’t know if he planned the whole development (Sieglinde getting angry and going to Ciel) and expected Liz to walk in on Ciel being over Sieglinde…

but he was at least expecting her visit and probably her initial jealous reaction to Sieglinde.

In comparison, in ch128…

Soma was on one side of the door when Ciel peeked inside and that’s when Soma punched him, thinking Ciel was his attacker. 

Seb was on the other side of the door so he couldn’t intervene right away and I’m thinking it’s not impossible that he was also slightly surprised by what had happened to Agni so he had a slight delay:

Another possibility is that, maybe you’re right, and Seb considered that Soma’s punch was probably not on the same level as Lizzie high-kicking Ciel in the face. So letting Soma punch Ciel in the face was no biggie (he just got bruised) and he made him pass out because he didn’t want Soma to be more of a problem.

Lastly, if we’re considering it from a narrative point of view, Lizzie hitting Ciel over a misunderstanding wouldn’t have been fair, so Sensei didn’t let it happen further than for comic relief. Meanwhile Soma managing to hit our!Ciel (his best friend) in the face is probably supposed to tell us readers that his reaction is a big hint as to who attacked him (the twin), so it was important to let it happen.

Ch107: *happens*

Friendly reminder that, in ch107, Seb is the one who told Lizzie that Ciel was back, just so that Lizzie would end up chiding him about not listening to Sieglinde….

…Like Seb initially tried to make him understand.