Downstream project invoked from trigger didnt lookup CI/CD variable from group

Summary

Downstream project invoked from trigger didnt lookup CI/CD variable from group

Steps to reproduce

Create 2 projects and define a CI/CD variable:

INFRA_CONTAINER_REGISTRY: gitlab.idp.edudom02.sa.gov.au:4567/d4e

1st project invokes 2nd project via the trigger keyword:

trigger_testrig:
  variables:
    SIFAPP_IMAGE: $CI_APPLICATION_TAG
  stage: package_test
  trigger: d4e/idp/sif/testrig/test

2nd project:

build:
  stage: build
  image: ${INFRA_CONTAINER_REGISTRY}/idp/sif/testrig/test-rig-base-image:latest
  script:

The GitLab runner errors with:

Error: InvalidImageName

Failed to apply default image tag "/idp/sif/testrig/test-rig-base-image:latest": couldn't parse image reference "/idp/sif/testrig/test-rig-base-image:latest": invalid reference format

But when running the 2nd project directly it works fine.

What is the expected correct behavior?

The 2nd project from the trigger should still be able to lookup CI/CD variables.

Using GitLab: 12.0.3-ee