This is a perfectly reasonable place to ask. And a reasonable question.
The answer is that the Mir default for fullscreen acts on a single output. (That is the common behaviour for window managers on desktops systems.)
It does make sense for mir-kiosk to support a “display wall” arrangement that treats multiple outputs as a single display area, but that would need code changes. The right repository to raise that request would be https://github.com/MirServer/mir/issues
HTH