going into side order, from the september direct trailer where Acht was first revealed i remember the joke at the time clearly being "and now Marina's ex is here".
the way this line [image description in alt] was written was basically the only evidence for this kind of idea, when the theories were kind of "Marina's order tantrum is sucking people in from her past and the DLC will be about going through her memories", so ellipses in a line like this is basically all theory crafters on no info need to go for shipping.
i'm not one who's super into plot theory crafting, i know full well the tendency to theorize something that's cooler than what you actually get and being disappointed that the story didn't live up to your imagination. the things i was obsessed with in side order promotional material was the obvious bleached coral theme, the symbolism of coral ejecting it that which keeps it safe out of stress being mapped onto Marina, the idea of her pushing those she loves (and those that keep her colorful) away out of a spiral (and it does turn out that was basically exactly what the prologue was going for)
so the whole "Acht and Marina exes" thing was kinda just a joke to me, wasn't even on my radar as something they were actually going to lean into, frankly i was still scared nintendo was going to make them kill pearlina by sending Marina to superhell or smth and we'd end up with a splatoonified destiel meme
so when the DLC comes out and it is legitimately a "they knew each other since childhood" thing, and the running bit is Acht feeling awkward third-wheeling pearlina, and it's explicit in text that one of the reasons they're coming back after the DLC is over is to scope out Pearl as the girl who took down the NILS statue who is now dating Marina... it struck me as really interesting.
at first it was me keeping up the "Marina and Acht are exes" as a joke, but as i kept reading dialogue lines, it slowly became less of a joke, they were to some degree dating because opposite but complimentary autisms, and then drifting apart as Marina got pulled away on the big girl assignment with DJ Octavio, and then the despair of knowing Marina left without even saying goodbye to Acht... it fits well into that reading, it slowly became less of a joke to believe that
but the thing that really makes me think this is intentional subtext is the final Acht diary entry you get from clearing Eight's palette. through the rest of side order talking about Acht's backstory, it seemed like they were retconning the OE lore that Acht had gotten themself sanitized intentionally, losing themself so they could explore their music deeper. but in the final diary, where Acht directly says they drifted into the deepsea metro to fall into their music, because, and i quote
Hey, Marina. You can guess the chaos your desertion caused. I ended up without much to do except make music.
they fell into a depression spiral when their girlfriend deserted their society without so much as saying goodbye, falling into their music deep away from interacting with everyone else, to the point that, as the old lore implies, they chose to give up their identity to escape the depression, but sanitization so thoroughly did it that they forget even making the choice.
so when they get brought out of that haze back into being themself again, with the only the barest strung-together horrified memories of what happened in the half a decade interim gap in their life, only to find themself replaced by some inkling they don't know at all, of course they're gonna be awkward seeing the two flirting.
they put on a stoic face because that's clearly their coping mechanism within this damaged body they barely recognize, hiding their eyes behind their tinted glasses so they can't be seen beneath. but the only time they let themself be vulnerable, the only time their eyes can be seen, is when they charge out in the climax when the world is at stake, diving in to try to save Marina, leaving the elevator and its protection behind to help the only person they remember ever caring about.
it's why i don't really like the aroace reading that much, because i think this reading is even more tragic and fits into the themes. the world has changed, it can't go back to how it once was, you can't put the octolings back in the canyon bottle. Marina abandoned Acht to the point they got their identity destroyed willingly to escape the pain, and when Acht came back they were replaced by the inkling whose voice they remember even through the haze of sanitized memories.
the lingering effects of sanitization have changed how they relate to everything (i think there's a fair argument to be made for the idea that sanitization took their gender can't have shit in the deepsea metro), but Acht clearly still cares for Marina and still, the slightest bit, resents having to be reminded repeatedly every time pearlina flirts in front of them how they were replaced.