What you're experiencing might be a form of PTSD, you're literally experiencing stress symptoms after exposure to a similar scene to the traumatic even that set the stage. The thing about PTSD is that it is not rooted in reason or logic and more in instincts and subconciousness; a hypothetical war veteran inflicted with PTSD might intellectually know that no one is out to kill him in his hometown, but that doesn't stop him from freaking out around construction sites.
In the same way, you acknowledge intellectually that, it's about time chuuni dragon brother gets his and the newly introduced female character happens to fit him very well. But what freaks you out might be the similarity of her and other female characters who is desperately thristing for MC right now. She is ambivalent if not slightly distrustful of MC, a rocky start to a relationship, if you will, a setting that is very much shared with the other heroines. Add to that her background, clothes design, relatively deep flashback and introduction, all unique qualities that makes her stand out from other supporting characters, the author signals she's here to stay, and not just some villain-of-the-week type character, which makes you subconciously put her into the same "Main Girl" folder as other heroines. Because of the other contents of this folder, you start to have a certain expectations that you're gonna get an arc where MC rizz her brains out like he did so many times before. Find out her secrets, solve her insecurities, cheer her hobby on...
Instead you get this side dude doing all the moves, and the girl falls for him.
In netorare literatures, there is a trope, where the girl has an interaction with MC1, this primes the relationship, and only waits for a trigger, this is followed by MC1 not doing shit while she gets railed by MC2, and MC1 remains blissfully unaware throughout the storyline.
The content of her arc up to now mirrors this, so this might have set off some alarm bells inside your head. If you happen to insert yourself as the MC, and at the same time, as a reader, you see that while he's doing nothing while the girl is being 'snatched' away by another man, you might reason that, because she have betrayed your trust, and went against what is 'best' for the MC (according to yourself), she have effectively betrayed the MC. And by proxy, the one romancing her, the chuuni, is invariably shuffled into the same lot as the stereotypical MC2 you see in netorare works, the evil ugly man out to steal the girl. This finalizes the equation, all the variables are there, the operations obvious, your brain did the math and the result is not what you like to see, so you freak out.
In this case, it does not matter that the chuuni guy is 'cool', that he's not out to steal the MC's girl, that the MC only superficially interacted with her, that the girl has her own life or something, there are a lot of logical excuses you can make, but in the end, your instinct has the final say: By your expectations the series, and by extensions, you, have set out for her, the new girl is supposed to hook up with the MC → She didn't, she went against that expectations → She violated your trust, from your point of view, the MC has one less girl than he would have had if she just played by the script → She betrayed him → This is netorare. → PANIC.
Honestly this is just my own take because I'm also affected by this chapter to a certain degree, despite it being objectively a very sweet chapter from a standalone perspective, in the grand scheme of things, I feel that the MC got fucking robbed by the author, for reasons unclear to me. For all I know, the author felt that the chuuni guy deserves a handout, so he did a simple -1 +1 operation at the expense of the MC, which is essentially netorare with author as MC2 and chuuni guy as the unwitting personification of the author.