It appears that the plugin-pom builds do not yet upload to Jenkins Incrementals.
This can cause downstream plugin PR builds to fail, with messages like this:
org.apache.maven.model.resolution.UnresolvableModelException: Could not find artifact org.jenkins-ci.plugins:plugin:pom:3.55 in incrementals (https://repo.jenkins-ci.org/incrementals/)
Example of affected PR: https://github.com/jenkinsci/mailer-plugin/pull/88
When I checked https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/ today, 'plugin' was indeed missing from the list.
If this is indeed the cause of the problem, it may not be sufficient to enable Incrementals uploads for future builds: it might be necessary to do one-off uploads for past versions too.