There's a certain degree of mystique surrounding Fitness Boxing, especially since a lot of people haven't even heard of it.
Although it seems like it's a full first-party title, it's actually an Imagineer project: it was only published by Nintendo in the west. The gist is that you'll use two Joy-Con to work out to boxing routines, with customizable trainers and "hit" songs that are mostly late 2000s and early 2010s joints.
Although it's unclear how it's doing in the west, over in Japan it seems to be pushing itself to the limit. Developer Imagineer notes on Twitter that Fitness Boxing has been a success for them, but "some shops are sold out." The account says that some stock is coming in over at Amazon Japan and asks folks to get the digital version off the eShop.
We'll have our review from our own CJ soon enough (he wanted to try it for a full month to see how it worked in that context). In the meantime this is just an interesting tale of how some games slip by in certain regions and are wildly successful in another.
Imagineer [Twitter] Thanks Jeff!