People forget that HP was/is pretty popular in Japan. Takeuchi even drew Hermione once lol.
Does Mash and Roman watching the 1984 Holmes series in Babylonia ep 0 count since it's not Doyle's books?
Otherwise, there's Vlad's thing about how Dracula is made up and ruined his reputation.
Both of those could arguably fall under "real in some form". Sherlock because it's just another adaptation of the original piece of "fiction" that turned out "real" (like so many other examples of "real" fictional Servants) and Vlad because the vampiric legends that largely gained notoriety only because of Stoker's Dracula actually affected the manifestation of his Servant form.
Wait til they casually reveal stroheim participated in the 3rd grail war.
Author fiat, mostly. It's a very loosely-defined system that most of the time boils down to "if a wizard says funny latin and waves his wand then magic happens" but also sometimes has complex self-sacrificial rituals because the plot demanded it. Specific details are hand-waved unless they're directly relevant to the plot, and half the time explanations are meaningless magibabble.
It's worth remembering that Rowling explicitly designed the rules of Quidditch to make sports fans angry. That's the type of author you're dealing with.
On the scale of soft magic to hard magic, it's definitely soft magic while Type-Moon magecraft is more on the side of hard magic. Note that there's definitely softer magic systems, though, and that magecraft is nowhere near the hardest it can get. Not for a lack of trying, mind you.
To give examples, a softer-than-HP system might be something like Narnia (where magic is entirely unexplained and merely serves the purposes of the plot) while a harder-than-TM system might be one of the various "magic is just poorly understood science (or sufficently advanced technology), actually" takes out there. Note that there are very few works that go fully hard or soft, and neither is better than the other. Personally I'm not much for the more sci-fi takes, though.
Seriously, though, you just need to look at the concept of "ranks" (or NP stats in general, really) to realize that the system usually fudges just enough lines to be able to bullshit stuff the plot requires. The fact that Mystery requires a lack of rational explanation is also a convenient out.
It's a hard magic system, don't get me wrong, it's just not exactly on the level where you start to get GURPS fans sweating. You don't have it written down that one unit of magical energy is 360 kilojoules, because you're writing a story and not an RPG system for math-obsessed nerds. (Writing what I know, here. That old usenet(?) gate argument was fascinating to read.)
Also Heroic Spirits as a whole kind of run on Narrativium, so, y'know. Fate is inherently a bit softer. Case Files mostly avoids this by focusing in on the more occult aspects of mages in general and leaning a lot on IRL mysticism (which gives it historical realism points if nothing else), but even then it tries its hardest to leave things fuzzy enough to not constrain future writers.
As one thing that always bother me in Harry Potter after I soaked in other fantasy stories (mostly japanese, mostly TM). That is, the magic expenditure. As I don't remember have seen something of this in all 7 books, that means they don't have it and you can spam Avada Kedavra non-stop?
I don't think it plays a huge role, but there are hints of certain spells taking a lot out of you. At least, I think there might be. Honestly, my main issue is that wizards don't use magic to strengthen their hand-to-hand like they do in Japanese series. It seems like such an obvious application, especially for law enforcement. Also, do even non-European wizards use Latin spells? What about pre-Latin times? We don't know.
I guess concept of "mystery" is where they dish out where plot demands but I find it reasonable considering it gives "each power is limited to a certain aspects". For example
Let's say Harry potter's Abra kadebra , a death magic it's defined as something that could kill anyone.
Any hax (hacks) can be overpowered.
Even though just simple mana blast could effect them any way. I like the concept but requires more explanation I guess