In previous gens, Spanish nature names only ever appeared in the
feminine form, so that's what we had. Now it looks like the isolated
nature names default to masculine — they're still feminine on the status
screen, but in the box you get "Naturaleza: [adjective]" and I guess
the adjective is considered to be on its own rather than modifying
naturaleza.
Italian doesn't follow suit so I guess it gets to be the one special
snowflake language where we don't have the default dictionary form for
natures.