A pet peeve of mine from Japanese writers is when they add a plot point, then immediately resolve it by saying "Little did they know, blablabla would happen".
The worst offender I've seen for it was from Nonbiri Nouka, where they straight up show a future scene of all the
dude's kids leading an army or some shit, talking about how great their dad is and patting each other on the back for being overpowered.
I'm genuinely sick of this, why bother having a story if you're going to tell me how it'll end before I've even gotten there?
Had to look up Nonbiri Nouka, and it does seem like that story also has similar low stakes vibes going by the tags.
Basically, intentionally spoiling that the story ends up with great success minimizes the stakes. A story that does it isn't going to pretend that anything dramatic will happen to the protagonist, so you should just expect a comfy ride of silly antics and funny characters.
For this story in particular, even if I've now been told that the dungeon will grow quickly and effectively, I'm still curious about how it'll get there. The queen seems like she'll be a fun character. What will happen when the secret gets out to the noble ladies and queens of other nations? The one dungeon core we know is a funny little gremlin, so what are other cores like?
Put another way, "spoiling the future/ending" very early just lets the reader know if the story is for them or not. If you feel like you were meaningfully spoiled and that there's no point in reading anymore, well, indeed, the story probably isn't for you.
Now, I'd be curious if you've read anything that wasn't a cozy, low-stakes story that still went with "spoiling the future." The first one I can think of off the top of my head is My Hero Academia with some early narration about how it was the story of Izuku becoming the greatest hero, which did start with an upbeat tone and a focus on introducing a large ensemble cast. And that large cast of characters became the way to sidestep the issue of knowing that Izuku will be fine since there isn't a guarantee that any of the other characters will be fine.