The option "Recurse in subfolders" (added with 1.18 in pull request https://github.com/jenkinsci/radiatorview-plugin/pull/11) does not work properly because it was just introduced as a workaround for a ClassCastException (
JENKINS-18025, JENKINS-20415, JENKINS-20664).
However, this feature should be enabled for users of the Folder Plugin. The behavior can be simply copied from the ListView(.jelly).
(A pull request is on the way)