由于@AllanHasegawa在另一期杂志Signing product flavors with gradle中的评论，我终于弄清了问题所在。简而言之，我必须在signingConfig null内添加buildTypes，因为Android添加了一些默认的签名配置。即使我试图覆盖它。根据我的问题的完整示例：

...
signingConfigs {
    release {
        storeFile file("../config/keystores/release_keystore")
        storePassword "mysecurepassword"
        keyAlias "myultrasecurealias"
        keyPassword "myreallysecurekeypassword"
    }

    debug {
        storeFile file("../config/keystores/debug.keystore")
        storePassword "mysecurepassword"
        keyAlias "myultrasecurealias"
        keyPassword "myreallysecurekeypassword"
    }

    other {
        storeFile file("../config/keystores/other")
        storePassword "mysecurepassword"
        keyAlias "myultrasecurealias"
        keyPassword "myreallysecurekeypassword"
    }
}

flavorDimensions "dim"

productFlavors {
    production {
        dimension "dim"
    }

    other {
        dimension "dim"
    }
}

buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'),'proguard-rules.pro'

        // this loop is a better implementation than my previous example
        productFlavors.all { flavor ->
                flavor.signingConfig signingConfigs.release
        }        
        productFlavors.other.signingConfig signingConfigs.other
    }

    debug {
        signingConfig null
        // this loop is a better implementation than my previous example
        productFlavors.all { flavor ->
                flavor.signingConfig signingConfigs.debug
        }
        productFlavors.other.signingConfig signingConfigs.other
    }
}