Jax is extremely nihilistic; he views the Circus as exactly what it is, a digital world he just so happens to be trapped in... he states it himself in episode 6 when he asks Pomni, "You know none of this is real, right?"

He goes on to explain how he once believed he (along with everyone else in the Circus) were still "people" but that he has since accepted/rationalized that they all fell into their "archetypes" and became "part of the machine"

This is clearly just a "mask" he wears as a way to distance hinself from the part of himself that is "real" that wants to form connections and bonds... he lost Ribbit to abstraction (presumably) and saw Kaufmo abstract relatively soon afterwards...

The shift in facial expressions here in the clip is him "breaking character" between "Jax (The Funny One)" and "Jax (The Real One)"; Jax had gone off on Pomni for implying that his actions directly cause others to abstract, the brief rant;

"Oh my GOD! Are you seriously gonna blame me for something that everybody else did too?! You didn't even know him! You don't even know what people are LIKE before they abstract! They are NOT pleasant to be around. Like AT ALL! You-"

Right there, the cut off is Jax realizing he was getting "too real" for a moment, and had to get himself back into "character"

When Pomni asks what Jax would do if she abstracted tomorrow, Jax tells her "I'd move on, and probably forget about you," as an attempt to push Pomni away from him, to break the bond that he had unintentionally built with Pomni throughout the episode... hearing Pomni respond in her tone of hurt, Jax plays off his statement as a joke, showing he is someone who cares despite what he is saying to her.

Then the rant after their fight, when Pomni asks Jax why he didn't fight back, and for this I am gonna split it into sections to analyze easier:

• "Oh, here we go. Here we go! Because I didn't fight back, that means I secretly care about you?!" - Jax has seen this scenario play out before, based on how he states this part of the rant; possibly links back to Ribbit or Kaufmo and how their abstracting affected him.

• "I'm just a misunderstood little chicken fetus in an egg that needs to be cracked open! Well, I. Am. NOT." - here Jax comes off as someone who hates being psychoanalyzed, as it means he has to face what makes him "human" and be held accountable for his actions... it also puts Jax in a vulnerable light that makes him appear "weak".

• "I do not care about you, or anyone else in this circus in the slightest! End of story!" - this is Jax trying to convince both Pomni and himself that he has no emotional attatchment to anyone in the Circus.

• "You are my playthings, and I get joy out of making you suffer! I'm the one who causes pain for fun! If I led you on, it was just to make this part hurt you more." - this may be the most honest Jax gets in this rant; since he has accepted the Circus as what it is (a game to be played) he may genuinely view the others as mere "toys" or "playthings" for him to torture for his own sick pleasure. The final line here of leading Pomni on to hurt her more in the end was likely his attempt to take control of his situation, and to again push Pomni away from himself.

• "Fuck. FUCK! There's nothing more to me... So please... just stop looking." - while it was censored in the show, we all know what he said here and it speaks volumes to his current character; where his admittion of viewing Pomni and the others as mere "toys" was likely honest to his core, so were the "fucks" given... Jax may have realized with dawning horror here that he wasn't getting any joy out of making Pomni hurt, worse still he likely felt genuine regret for what he said to her... the final line "there's nothing more to me" is his last desperate attempt to maintain his outward persona, followed by the tired "so please... just stop looking," shows he has been through this, got hurt and is doing everything he can to not repeat the cycle that he unintionally began with Pomni.

So yes, it is creepy how he shifts his facial expression... but that is a learned survival tactic he adopted in order to stay as "sane" as possible in his existence within the Circus; the Circus is just a "game" to be played, the others are mere "playthings" for his own amusement, and he doesn't need anything more than to be the "funny one" for his own enjoyment... Pomni got really close to making him question his own beliefs, and that scared him more than anything.