How These Historical Mysteries Work
Okay, so I've read dozens of these things...dozens. And the dozens and dozens of the adult historical mysteries set in the nineteenth century that I've read involve a female/male couple, who meet angry, end up with lots of sexual tension, and, after a few books, sex. At least one part of the couple is either tormented or has some torment in his/her past. One or the other of them ends up in physical danger that the other one saves them from. As a general rule, the characters are of more interest than the mysteries. And, in my experience, many of these series do the literary equivalent of jumping the shark at some point, and the characters and the sex aren't enough to keep you reading.
"A Spy in the House" And The Historical Mystery Pattern
A Spy in the House pretty much follows the pattern for the first book in one of these series. The female lead has a rough background, she's in conflict with the male lead, at least one of them ends up in physical danger and the other one saves the day. The ending may or may not be typical of these series. I'd have to read the next book to see.
A couple of times references are made to the leads' ages. The male is twenty, the female seventeen, but that's pretty much the only way we know how young they are. At the halfway point, something comes out about the female character's family and background that adds a bit of search for identity to the book world, something that is YA-ish. But it isn't a major part of the rest of the story. The sexual tension isn't very high in this book, though there's enough to suggest something could come of it down the line. This lower key love interest could be a nod to the YA audience.
Otherwise, this is pretty much an adult book with teen characters. The teens could be pulled out and replaced with adults with very little effort.
Nothing Wrong Here
A Spy in the House isn't the first book I've seen that works like this. Au Revoir Crazy European Chick from 2012 has an adult-like character in an adult-like thriller geared toward teen readers. I Hunt Killers, also from 2012, is a teen Dexter. Illuminae from 2015 is a science fiction space thriller in which teen characters definitely function in very adult ways.
There's nothing wrong with any of this. We're just not talking books with traditional YA themes and YA situations. At most there may be a loose connection to a missing parent to tie the books to the genre. Otherwise, you could imagine any number of adult actors playing these characters in the movie adaptation.
And that is probably the point. These books give teen readers an opportunity to imagine themselves in far more adult and thrilling roles than those they find in more traditional contemporary YA books.