We found the culprit. Just saw in a small print, "Standard colour is not available on Amazon.co.jp or Amazon.com.au. If selected, your book won't be printed in those marketplaces."
We'll look into it and maybe create a different version of paperback or hardcover. We're still working through the details but maybe premium colors would work. Anyway, we will let you know if there are any updates regarding this. We might use another publisher like Lulu too.