After upgrading to Jenkins 2.93, ThinBackup sets the permissions of installedPlugins.xml to 600. Previously, the permission were 644, matching the original file.
If I watch the permissions as a backup is running, the permissions start as 644, but change to 600 by the time the backup completes.
The dnusbaum user on IRC recommended I create this issue.
EDIT: This issue was discovered when automation attempts to copy the backups files off of the server as non jenkins user. I assume since the permissions of the backups are different than JENKINS_HOME, that restoring would fail. I have not had a chance to test this theory.