It’s the same way dramatic irony works. The readers know something’s up while the characters remain ignorant, which adds tension as we brace for an impact we know is coming. Shifts the tone a little, gives us a heads up so twists and changes don’t feel like they’re cheaply coming out of no where, and makes us wonder what exactly is going to happen that makes it all go downhill.
I'd argue it does the opposite of all of those things. It's just flat out "here is how it is", so you know all of those things in advance - there's no twist or change in the first place since the direction has already been announced.
At most it adds this fake tension you speak about, but that's rarely better than tension of the unknown. It's not like readers (as demonstrated in this thread) are so blind that they don't see that something is going on here without having it spelled out ...
Just because you don't like it doesn't mean it's invalid. It's a time-tested narrative mechanism for centuries, if not millennia.
Also, for much of history, "tension of the unknown" wasn't much of a thing, especially with regard to the ending, unless maybe you were a kid hearing something for the first time. On re-tellings, you wouldn't even have "tension of the unknown" with regard to the narrative path, either. The modern obsession with "avoiding spoilers" is not universal.
The modern obsession with "avoiding spoilers" is not universal.
That's not even a thing. Misconception.
People love spoilers, for the most part. They won't admit it (fashionable contrarianism and all that), but research and focus group testing shows that to be the case. Movie trailers that have clear spoilers are more effective at getting people to come to cinema and talk about the movie before it is released. Romances often signal clearly who the true main couple is, no matter how insane the story itself turns out to be and how many twists there are. Isekai protagonists are usually practically invincible with plot armor, if nothing else, and are rarely put to a genuine risk and losing something of real value.
Only place where spoilers are considered bad are real mystery stories, where the whole point is to try to figure out whodunit or what is going on. If we're talking novels or manga, things like Re:Zero go to this pile, or romances that have a genuine love triangle with two fairly legitimate options, resulting from something like amnesia or deep-seated trauma that is incredibly hard to get past of. For yuri stuff, that's something like Letink's "Re-Blooming".
They won't admit it (fashionable contrarianism and all that), but research and focus group testing shows that to be the case. Movie trailers that have clear spoilers are more effective at getting people to come to cinema and talk about the movie before it is released.
Definitely need more of this. Aggressively masochistic girls going after inexperienced older women is a tiny niche that I love and never see (probably because it's so niche). I very much look forward to updates!