See https://phabricator.wikimedia.org/config/issue/config.unknown.gitlab.api_key/
cf T324149 and especially T385480 mentioning gitlab_api_key, different spelling than gitlab.api_key (dot vs underscore), same in puppet's modules/profile/manifests/phabricator/main.pp.
Phab admins can see its current value in the table on https://phabricator.wikimedia.org/config/edit/gitlab.api_key/
Maybe that Phab local config value predates pulling a secret value from Hiera?
Not sure I interpret T385480 correctly thus CC'ing @brennen for potential input.