Additional info that might be related. The cluster is deployed into Elastic Cloud and it has the "User setting overrides" specified as:
xpack.notification.pagerduty:
account:
api_service:
service_api_key: <service-key>
other_service:
service_api_key: <service-key>
that should be appended to elasticsearch.yml file when Elasticsearch nodes are being started and, according to https://www.elastic.co/guide/en/elastic-stack-overview/6.6/actions-pagerduty.html#configuring-pagerduty it should still be valid.