Resolution: Not A Defect
Debian/bullseye, Jenkins v2.361.2
Jenkins plugins manager reports that https://plugins.jenkins.io/cloudbees-folder plugin version 6.758.vfd75d09eea_a_1 is currently installed (and /var/lib/jenkins/plugins/ cloudbees-folder/META-INF/MANIFEST.MF confirms that), but version 6.770.ve57b_a_fb_6a_67c is reported for update, though as `Unavailable`. (See attached screenshot)
Also https://plugins.jenkins.io/cloudbees-folder/ reports version 6.9 as latest, though at https://updates.jenkins.io/download/plugins/cloudbees-folder/ the 6.770.ve57b_a_fb_6a_67c is listed as latest one.
Something feels strange here?
(I have seen this behavior on multiple Jenkins installations in the meanwhile, so it might affect more users?)