Alhaitham and Kaveh's dynamic is interesting, because I've seen a lot of people say either "oh my god Kaveh is so mean" or "oh my god Alhaitham treats Kaveh terribly". But, quite importantly imo, in this quest Paimon remarks that she "doesn't know which one of them is more problematic".
I think when considering their relationship you need to look at intention. Do Hoyo want to villainise one of their popular husnandoes as a toxic asshole? No. Even the "villain" playable characters like Childe and Wanderer are nice/funny on occasion. Kaveh and Alhaitham's interactions are intended to be humorous, to make you gasp and say "oh wow I can't believe he just said that!".
Like, during the most recent story quest, for example. Kaveh accusing Alhaitham of having no empathy when he's clearly pretty emotionally stunted and decently moral is mean. But saying "At least I'd be a fungus with empathy!" Undercuts that - it's clear neither Alhaitham nor the player are suppose to take those words seriously.
Alhaitham dismissing Kaveh's field of work with "you work yourself to death just for a smile" is so belittling, but it's a retort to Kaveh saying his scribe job is at "rock bottom" in the Akademiya. Alhaitham isn't lashing out or trying to demean Kaveh in front of others (this interaction happens after the player has "left"); he's just continuing their verbal sparring match. I think people take this jab so seriously because it can be seen as a jab at a lot of people working out of passion, but in the context of the dialogue we're not supposed to regard it so deeply.
It's also fairly clear that nobody in game takes them seriously either. Tighnari says Kaveh is "as pent up as an anemo slime" in his Alhaitham voiceline - another funny quip - and Collei details how hard it was to suppress her laughter when Kaveh was talking about Alhaitham. Nobody thinks they're genuinely bad for one another, or that getting them separated is all that crucial.
Even the interactions in this story quest are just so silly and domestic: Kaveh can't get the painting straight, Alhaitham buys ugly furniture, they're arguing about rent and drink tabs, Kaveh is doing an unfair amount of housework and Alhaitham fucks off without entertaining his own guests. it's funny, or at least it is clearly intended to be.
I've heard that in the other dubs the VAs tones are different and it makes the lightheartedness clearer. Personally I've thought since their first interaction in the AQ that Alhaitham is having fun during their quarrels, and Kaveh is just melodramatic enough to act traumatised even if he isn't truly offended.
Then when it comes to whether their dynamic is supposed to be read platonically or romantically... Well Hoyo won't do anything canon, but it's hard to argue that there is no intentionality in how their relationship is portrayed. What other character(s) has an idle animation directly referencing their relationship with another character? How many other voicelines explicitly reference two characters as a pair? Nahida's Kaveh voiceline, Collei's, Tighnari's - I'm sure there are more. "Roommates" "senior & junior" "academic rivals" -> certainly there's a case to be made that they're just very close, but Kaveh and Alhaitham getting fifteen minutes of epilogue interaction in two different quests (where none of the content is related to either quest) says something. Fanservice, quite possibly.
Also Kaveh's line "We used to be friends, but not anymore"; I've seen plenty of people interpret this as "oh we're not friends, we're dating*". I think this is fun in fanon, but on a canon level it's basically a promise by Hoyo to fix (or explain) their current dynamic. Hoyo, for all their many faults, are detail orientated enough to follow up on such a line. Perhaps it will just be in Kaveh's voicelines/character stories, but I anticipate a future event or Kaveh hangout in relation to their bond. Nothing explicit, but more exploration. It's fairly clear Alhaitham and Kaveh are popular enough characters to warrant it.
All this to say: I have lots of thoughts. I have seen people call them toxic and (while idm the drama of that in fanon shipping) that isn't really the intended case in canon. Or, at the very least, the intention is not to to take their squabbling so seriously.
