The problem with this release lies in the update procedure. If external connectors are configured in Elasticsearch and Elasticsearch is then updated to 7.12, these connectors are removed instead of migrated to the new version.
This affects connectors to external applications, often used in combination with Atlassian's Jira, ServiceNow, IBM's Resiliant, Slack and others.
The bug also affects connectors used for actions and alerts which are triggered from within Elasticsearch and then connected to an external application.
Users using external connectors should not upgrade to Elasticsearch 7.12. Instead wait for a bug-fix release, which will likely be the upcoming release, 7.13.