Sanju ko tokne ke baare mein agar tum first time ki baat kar rahi ho (jahan par Sanju-Sid-Faizi hote hain and Sid-Faizi convinced hote hain ki Karan is acting weird because of the proposal) then mujhe lagta hai ki the primary reason Faizi didn't think twice about it was because he himself was excited.
Another reason being that he probably doesn't expect Sanju to understand ki Karan ke liye Monami ko propose karna kitna mushkil hai.
Agar tum second time ki baat kar rahi ho (when they are in Batra Sir's cabin, after Karan spills the water on the laptop) so the reason might be that Sanju straight out said 'Yeh Karan Sir ji hi naa lag raho hain.' The idea is so absurd that Faizi doesn't even give a second thought to it. It's like saying, SRK is in my porch. Technically possible, but so implausible that I won't believe it.
If Sanju had just pointed out what weird things she had noted, Faizi might have paused and thought about it, and it would irk him, but since the idea she proposed was so absurd, he just brushed it off as her being overly suspicious.
It's like, going to your Mom and saying "Mom, I think I have diabetes" because you did a google search and have 5 symptoms of diabetes but you're a low risk person. Your Mom won't think twice before brushing you off but if you go to her, and list those 5 symptoms, she might pause and think about them, and wonder if it's anything serious.
As for Faizi understanding how weird Karan would act, Karan already had A LOT on his plate. Karan is tensed about Baby-Bhabhi's safety, Monami's safety, the academy's security, proposing to Monami, what Dhanu is planning, not being able to catch Dhanu, Param Bhaiya's barsi and the accident, so all that is just adding up to a very weird set of habits.
He has 4 very sensitive points- Baby, Bhabhi, Param and Monami. He's going to act all weird when they are involved. And they are ALL involved.
Having said all of that, it does make sense from his character's POV why he isn't suspicious, but I kinda wish Faizi would be the one who takes the charge on it, rather than Sanju. I wanted to see smart agent Faizi. I want to see Faizi-Koel, both as a couple and as individual characters.
Kavya, hum itna kaise agree kar rahe hain? Kya kuchh gadhbadh hai?😆
I have already lost the title of your A2D bro, toh kuchh aur sochna padega...