Issue with variable type when referencing

[aykhans]

In the build-all task, when I reference the GLOBAL_ARCHS variable, it passes the ARCHS variable to the build-platform task as a []interface{} type.

version: "3"

vars:
    PLATFORMS:
        - os: darwin
          archs: [amd64, arm64]
        - os: freebsd
          archs: [386, amd64, arm]
        - os: linux
          archs: [386, amd64, arm, arm64]
        - os: netbsd
          archs: [386, amd64, arm]
        - os: openbsd
          archs: [386, amd64, arm, arm64]
        - os: windows
          archs: [386, amd64, arm64]

    GLOBAL_ARCHS: [386, amd64, arm64]

tasks:
    build-all:
        silent: true
        cmds:
            - for: { var: PLATFORMS, as: PLATFORM }
              vars:
                  OS: '{{ .PLATFORM.os }}'
                  ARCHS:
                      ref: .GLOBAL_ARCHS
              task: build-platform

    build-platform:
        silent: true
        cmds:
            - echo {{typeOf .ARCHS}}
            - for: { var: ARCHS, as: ARCH }
              cmd: echo "Building for {{ .OS }}/{{ .ARCH }}"

However, when I reference PLATFORM.archs instead, the ARCHS variable is passed to the build-platform task as a string type instead of []interface{}.

version: "3"

vars:
    PLATFORMS:
        - os: darwin
          archs: [amd64, arm64]
        - os: freebsd
          archs: [386, amd64, arm]
        - os: linux
          archs: [386, amd64, arm, arm64]
        - os: netbsd
          archs: [386, amd64, arm]
        - os: openbsd
          archs: [386, amd64, arm, arm64]
        - os: windows
          archs: [386, amd64, arm64]

    GLOBAL_ARCHS: [386, amd64, arm64]

tasks:
    build-all:
        silent: true
        cmds:
            - for: { var: PLATFORMS, as: PLATFORM }
              vars:
                  OS: '{{ .PLATFORM.os }}'
                  ARCHS:
                      ref: .PLATFORM.archs
              task: build-platform

    build-platform:
        silent: true
        cmds:
            - echo {{typeOf .ARCHS}}
            - for: { var: ARCHS, as: ARCH }
              cmd: echo "Building for {{ .OS }}/{{ .ARCH }}"

Why does referencing PLATFORM.archs pass it as a string type, while referencing GLOBAL_ARCHS pass ARCHS as a []interface{} type? And how can I modify it to pass as []interface{}?