Cannot load grand-child routing from child module

Play Version

PlayScala@2.7.2

JDK (Oracle 1.8.0_72, OpenJDK 1.8.x, Azul Zing)

$ java -version
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (Zulu 8.38.0.13-CA-linux64) (build 1.8.0_212-b04)
OpenJDK 64-Bit Server VM (Zulu 8.38.0.13-CA-linux64) (build 25.212-b04, mixed mode)

Expected Behavior

1. make your sbt project as 'root'
2. add sub-project as 'buildA'
3. add sub-project as 'appA'
4. buildA include a appA.routes
5. -> Yay, you can create some builds in one root project !!

Actual Behavior

My child-project (buildA) can NOT load grand-chid-project(appA)' s routing.

$ tree -L 2
.
├── build.sbt
├── build-app
│   ├── app
│   ├── build.sbt
│   ├── conf
│   │   ├── application.conf
│   │   ├── logback.xml
│   │   └── routes
├── core
│   ├── app
│   ├── build.sbt
│   ├── conf
│   │   └── core.routes
│   ├── src
├── project
     ├── plugins.sbt

build.sbt

lazy val buildApp = project.in(file("build-app"))
lazy val root = project.in(file("."))

build-app/build.sbt

lazy val core = project.in(file("../core")).enablePlugins(PlayScala)
lazy val buildApp = (project in file("."))
  .enablePlugins(PlayScala)
  .dependsOn(core)

build-app/conf/routes

GET     /                           controllers.app.HomeController.index
->  /core core.Routes

core/conf/core.routes

GET     /                           controllers.core.HomeController.index

$ sbt "project buildApp" compile
[error] /home/sizer/go/src/github.com/sizer/hello-sbt-multiproject/build-app/conf/routes:3:1: not found: value core
[error] ->  /core core.Routes

Can NOT load core.routes :sob: Am I Wrong or Is it correct behaviour??

my project is below.

https://github.com/sizer/hello-sbt-multiproject/tree/playframework_failedExample