But my point is: if she's established as a potential traitor in-game, whatever the mechanics of that could be, Shachiku should've been a lot more wary of her acting weird and upset that day, and he should've immediately thought about this potential outcome instead of just thinking about the reasons for the weird behaviour.
Him still being too fixated on existing game events and being unprepared for new "real game world" interactions is why it was fine before and why it's so stupid now that this has been confirmed as an existing game event, something that he should never miss in a million years, the way it's been established so far.
I suppose one reason is that he doesn't really
do 100% runs anymore - correct me if I'm wrong, but I'm pretty sure it was established pre-isekai Shachiku usually does Any% strats, which is what we've majorly seen thus far.
We also need to remember Shachiku is witnessing a
bunch of shit that's never happened in his playthroughs, cause the "game" is
now real, essentially. Ada's brother, some of the inner specifics regarding that whole Tesset arc, a couple more details I'm likely forgetting because this manga is 3-years old and is running on a (bi?-)monthly publication and I haven't bothered to do a reread... that's not even getting into his goal of figuring out what the heck Rulebreaker even is
to him, since thus far only we sort of have a grasp on it as audiences.
It needs to be stated that we, as humans, tend to forget things when we (re)process information, or at the very least put them on "lower" priority. Given the absolute whirlwind of shit The Gang had to face right before Brahms showed up, as well as the onslaught of information gathering that happened right before this sudden detour, I think it's very possible Shachiku straight up didn't even begin to think about any of that, especially since thus far the route, to him, is still on track despite new roadblocks cropping up.
Now, does it absolve the writing's quality here? Not really. I think a few lines about him finding Brahms sudden retreat and Kiira's blood being practically everyone should've had him ringing some alarm bells as something to prep for, or maybe a "shit, I was so focused on other things I didn't connect the dots sooner" card could work considering what I said earlier, but either way, what's done is done, and (hopefully) the rest of this arc can help pick it back up. And, ya know, at least it's nice to read someone's critiques on this development that
wasn't just letting the inherent bias about a trope clouding everything within the plot.