Extends doesn't expands variables correctly

Summary

When using extends with a variable definition (either for the script or the ones within the project) the variable redefinition is lost. In contrast, a YAML anchor performs the job correctly.

Steps to reproduce

variables:
  LOCAL: "global"
  ANCHOR: "global-anchor"

.echo:
  script:
    - echo "CI_REGISTRY_IMAGE=${CI_REGISTRY_IMAGE}"
    - echo "LOCAL=${LOCAL}"
    - echo "ANCHOR=${ANCHOR}"
  
.my-var:
  variables:
    # this should be defined within the project variables
    CI_REGISTRY_IMAGE: "${CI_REGISTRY_PATH}/tf"

.local-var:
  variables:
    LOCAL: "local-var"

.local-anchor: &local-anchor
  variables:
    ANCHOR: "local-anchor"

build:
  stage: build
  variables:
    VER: "latest"
  extends: .my-var
  extends: .local-var
  <<: *local-anchor
  extends: .echo

For several levels of extension and redefinition check https://gitlab.com/-/ide/project/adin/variable-test

Actual behavior

The variable redefinition is lost while using extends but it works as intended when using YAML anchors.

Expected behavior

The variable should be redefined when calling an extends that redefines it.

Relevant logs and/or screenshots

Compare the output of https://gitlab.com/adin/variable-test/-/jobs/130977198 the jobs when printing the variable ANCHOR, LOCAL, and CI_REGISTRY_IMAGE. Anchor uses the YAML anchor mechanism and is redefined. However, the other two are not and they use extends.

Environment description

Using gitlab.com. Check the repo with an example https://gitlab.com/adin/variable-test

Used GitLab Runner version

Running with gitlab-runner 11.5.0 (3afdaba6)
  on docker-auto-scale fa6cab46
Using Docker executor with image busybox:latest ...
Pulling docker image busybox:latest ...
Using docker image sha256:59788edf1f3e78cd0ebe6ce1446e9d10788225db3dedcfd1a59f764bad2b2690 for busybox:latest ...
Running on runner-fa6cab46-project-9801401-concurrent-0 via runner-fa6cab46-srm-1544119481-c23baa8d...