After an upgrade to version 2.249.1 of Jenkins core and version 9 of the Analysis Model API plugin (both were done within minutes of each other), we could no longer view Checkstyle issues from builds that were run with the old versions (Jenkins v2.235.5 and Analysis Model API v8.3.1), receiving the following unmarshalling error:
I've attached a more complete log with stacktrace and slightly anonymized versions of the Checkstyle XML files that were being read.
I have worked around the issue for the time being by downgrading the Analysis Model API plugin to v8.3.1, which has allowed existing builds to display their warnings again. We did not have any builds run with the newer version of the plugin, so I cannot comment whether new builds would have their warnings displayed while old builds would not, but this is not an ideal situation even if it's the case.
Given it is now working with Jenkins 2.249.1 and Analysis Model API v8.3.1, I suspect an issue in v9.0.0 of the plugin and not in the Jenkins upgrade.