This is really weird: 

/tellraw @a {"translate": "painting.minecraft.void.author"}

gives

Kristoffer Zetterstrand

... as it should. But

/tellraw @a {"translate": "painting.minecraft.wind.author"}

gives

painting.minecraft.wind.author

as if it was undefined. But it is defined, as "Mojang", as far as I can tell. I discovered this because it fails for my replacement paintings, for all the unused paintings, but not the other paintings. And this works for title as well, it's just (afaict) the artist.