Every Japanese Wondercard has this issue with the "1". I don't think there's something wrong with the images.
Rather it seems like that is a general problem in the Japanese games. As far as I know the font isn't even in the WC, it's in the game itself.
Even if you insert an English wondercard, where all letters and numbers are half-width, into the Japanese game, the date will appear full-width. Except the "1" which will be half-width again.