It does not appear to be the same situation as
https://issues.jenkins.io/browse/JENKINS-65585, and yes, I've tried @POST. What I thought might be happening is that the validation methods are in the base class, but that appears not to matter. I've checked the spelling pretty carefully. It would be nice to know exactly where the resolution of the check is done, because single stepping through the Jenkins run-time is not providing any wisdom. The check is not being invoked/found after Apply on a freestyle project or when configuring the plugin. Getters and setters are all fine, but the validation methods for any fields, including the ones that used to work, seem to be ignored. I can't revert to the version of Jenkins where this was last known to work definitively (around 2.74) because of dependencies.