Looking at the task definition on AWS ECS, they look to have been set properly

"environment": [
    {
        "name": "REACT_APP_FEATURE_ARTIFACT_TABLE",
        "value": "1"
    },
    {
        "name": "REACT_APP_FEATURE_ARTIFACT_SEARCH",
        "value": "1"
    },
    {
        "name": "REACT_APP_FEATURE_RUN_GROUPS",
        "value": "1"
    }
]

It could be because the dockerfile doesn't export these ENV vars https://github.com/Netflix/metaflow-ui/blob/master/Dockerfile

So what finally worked was using

FEATURE_*

instead of

REACT_APP_FEATURE_*

. The final set of vars are, no

extra_ui_static_env_vars

are needed.

extra_ui_backend_env_vars = {
    FEATURE_RUN_GROUPS      = 1
    FEATURE_ARTIFACT_TABLE  = 1
    FEATURE_ARTIFACT_SEARCH = 1
  }