They didn't have people doing scanners for this stamp (they told me they weren't working that day), but did it always say you'd get a key for meeting characters? I wanna say that's new, because I could have sworn that back then, the objective description just said "Meet [Character]". It now has the mention of the key on the Mario Bros. and Peach now.
Either way, that's not the only new way to get a key:
I think this is a good addition (especially if you get keys for meeting the characters now), might help take the pressure off the minigames. Makes me wonder if it's possible to farm the 8 bit keys, which I didn't get to check.
Lastly, there's this:
Don't know if this was already in Japan, but at least this confirms they can add more stamps in the future.