Let me preface this by saying I don't have any problem with Tsukasa's manners or lack thereof. He's totally fine the way he is. (And 'manner' probably isn't the word I'm looking for, but...)
I've seen KR Decade up to episode 27, and probably will cover a few things up to that point, but I will try not to spoil, and certainly not any important details.
Anyway, it's probably nothing that just happened recently, and not even remarkable, but I've been noticing lately (as in the latest episodes I watched) that Tsukasa seems to lack outward (or any) displays of common courtesy. I wouldn't call him rude exactly, but rather impolite. From what I understand of acceptable Japanese conducts (my understanding could be wrong, of course), you greet people, or at least incline your head, when seeing someone; you say sorry when bumping into someone; you say thanks for the food or anything you are given (either a favor or an object). I just don't remember Tsukasa ever did any of those things. And he never really addresses anyone with -san either. (But I don't mind. As I said, I don't want him to change or anything.)
He certainly isn't the first (nor the last) rider who doesn't do the basic courtesy thing, but it just seems to stand out all the more, since Yuusuke and Natsumi, who often travel along side him, do all of 'acceptable conducts' above (well, at least Yuusuke does) and Tsukasa just sails right past, as if those basic manners have nothing to do with him.
This is probably me reading into it too much, but while I don't think Tsukasa (the current amnesiac Tsukasa--I know virtually nothing of his past, and prefer to find that out by myself) actually hates the world, I think he does have certain disdains for the way human act, with all the politeness that's more often than not turn out to be fake, or a mask of something more sinister. Not coincidentally, that sickeningly polite world where Daiki comes from makes these all the more obvious.
Okay, what I really want to say is I think the proper response to 'okaeri nasai' is 'tadaima'. But since Tsukasa doesn't seem to have much faith in the common polite stock phrases (really, the only one I ever heard him say is 'yoroshiku'--and I think he only says it because it would be weird if he doesn't), we get nothing. Still, I appreciate that the show was actually attempting to explain that scene, even though I feel like I've seen a similar scene before (UM Tiga--from where I took the tadaima line) (perhaps a spoiler for UM Tiga). But knowing what to say doesn't matter either way since I can hardly imagine Tsukasa saying tadaima, exactly.
Well...as an aside, since I'm heading in that direction, I have always had a soft spot for canon couples (and especially for this particular couple, whom I was sure about from the beginning, even if they themselves (especially Tsukasa) were not). Still, I was totally caught off guard by that corny as hell scene in 27; that's the stuff I hadn't even previously considered putting in my private fantasy. I'm probably the only one who loves the scene, though, or finds it remarkable in any way, since it isn't in any doubt that things will turn out fine. Guess I'm a hopeless romantic; for me, actions always speak louder than words. (Speaking of which, I'm kind of amazed that Tsukasa has corny speeches for practically everyone, but strangely none for himself.) In any case, would it be too much to hope that the consequences of this scene would be touched on in later episodes (three left, I think)? I think I'm mostly holding off on 28 because I'm afraid the incident would be swept under the rug, never to be referred to again. Like, 'who cares if the show just spent most of the two previous episodes for that? (And the guests were more interesting than the main casts anyway.) Now that everything is well again, let's move on.' (Turns out the scene is recounted almost entirely in the 'last time on KR Decade' segment, but I'm really scared to watch past that at the moment.)
P.S. I love the Kasumi no Joe outfit (wish he spends the entire arc in that outfit, not that the other one doesn't look nice). Makes him look totally like a game character ...though I'm not sure which game...(a fighting one, I think, but not a rider one). In any case, I regret that I remember almost nothing of Black and Rx, including who exactly is Kasumi no Joe. (I saw the shows once around 4-5 years ago.) Glad that they got the original Kotaro Minami back, though, even if he seems overshadowed (screentime wise) by the villains...