Changing JENKINS_DEBUG_LEVEL to either "1" or "9" does not affect log level. With either, I still see the same logs, INFO level logs.
Jenkins v2.190.1 on CentOS 7.3
This is probably because the option does not do what you'd think (and what it looks like): It's a simple wrapper for the --debug option for Winstone-Jetty, and only controls that component's logger:
https://github.com/jenkinsci/winstone/tree/master#command-line-options
It has no effect on messages logged by Jenkins.