So this is something I've been thinking a bit on recently, but I have the feeling that Pokemon Legends Arceus did a lot better than Game Freak expected. For a main series game (which it is), it has a very odd combination of pre launch build up and post launch focus in stuff like the anime and stuff, and I legit think they were expecting something more akin to a Mystery Dungeon than something like what they got. 

Of course I'm a narrowly focused sort of game, and I'm curious about some other hits that came out recently that you think are like that. Let's set recently to...2017-2022 for a five year period of time for reference

I would say Breath of the Wild. I'm sure they expected it to be a success, but I don't think they expected it on that level.

Mario and Rabbids Kingdom Battle. It's not like Rabbids are a selling point, and Mario and cover based srpg was a seemingly bizarre choice.

Clubhouse games. I think this was kind of testing the waters on how such games would be received on Switch, which is why it came out before the more obvious choice of Switch Sports.

Street Fighter V. It got off to such a poor start that it seemed like it would go the way of MVC Infinite. However they were able to correct course. Speaking of which...

Marvel vs Capcom Infinite. In the other direction. With the MCU at its Zenith this seemed like a surefire hit. Unfortunately some bad design choices, especially with the roster, made it dead in the water. A shame because it's really not a bad game.

Sort of. Depends on the frame of reference. I think if we looked at things from our 2017 vantage point, we'd have been very surprised. But, after the sales of BOTW, Mario Odyssey, Super Mario Party, etc, I think we understood the trend. By the time Metroid Dread came out, I think people expected it to be the best selling Metroid game, which I'm pretty sure it is or will be.

Dreamland is the only one it may have difficulty passing, but I think it will beat it pretty cleanly.

Persona 5 probably surprised Atlus. They seemed surprised enough about the Steam version of P4G.

Amazon Games with New World. Their first MMO and a successful one at that. Was super popular at launch and for the couple of months despite many set backs and middling reviews. Still has an active community going for it.

Elden Ring by From Software sold 12M in less that a few weeks (and by far their best selling game), got 95+ in OC and MC. They even stopped reviewers like Digital Foundry from posting their review pre patch due to concerns of performance pre launch. Very much doubt that they'd imagine the game blowing up the way it did. Still over 500K concurrent players, peaking at >600K on Steam which is insane.

God of War (2018) by Santa Monica Studio sold over 20 million on PS4. Several times higher than its predecessors and highest reviewed in the series. Coming from the excellent trilogy, and almost re-imagining to the series I don't think SMS expected to be as popular as it is now. Sold over 2M on PC iirc as well.

If I just limit this to Nintendo IP:

BotW
SSBU
Mario Kart 8 Deluxe
Animal Crossing: New Horizons
Pokemon Legends Arceus
Super Mario Party
Splatoon 2
Luigi's Mansion 3

I know there are a lot of other Nintendo IP that happen gotten a boost on the Switch, but these are the games that have sold so far beyond their predecessors that they had to have beaten expectations. (Or in the case of Arceus it was a spinoff that has far exceeded the sales of other Pokemon spin-offs.)

Nier Automata.

Nier went from a spinoff of a series of janky games in Drakengard and Nier itself was Janky but good ideas and a small but dedicated fanbase. It was a miracle it got a sequel at all and that sequel sells close to 5 million copies. Becomes a meme with 2B. Cross over with Final Fantasy 14 and Gravity Rush which led to Nier getting a remake with Automata's combat. Some smaller spinoff games on phones. A lot of 2B merch and cosplays. Now Nier is a successful series for Square.

Ring Fit Adventure, Astro Bot: Rescue Mission, Celeste...

I feel like Tales of Arise surprised everyone. Given the production quality they must've had some faith they could take the franchise to new heights, but I'm sure it still worked out better than expected.