Yesterday, JFrog support changed something in Artifactory and I was able to copy Jenkins 2.289.2 and 2.300 to the expected releases repo.
Rather than a few seconds, the copy operation took about an hour however, and between 11 PM CEST yesterday until around 9 AM CEST today, the maven-metadata.xml for the various core-related artifacts claimed that 2.289.2 and 2.300 were the only releases ever. If you've seen weird behavior during that time, that may be the cause.
I'm currently reverting all the various workarounds: The camilla and releases-20210630 Maven repos are no longer available; The jenkins-infra/jenkins.io
, jenkinsci/docker, and jenkins-infra/update-center2 workarounds have been reverted.