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I only can confirm that it is working with grails 5, and 6. I used pretty much its default configurations. It worked just fine. Maybe you can try it without any customized configurations first.

We're trying to eliminate environmental changes. What version of Java are you using? Don't you need to specify the packages to monitor?

i used 11, and now 17(as I already moved to 6). I did not specify the packages, I believe it comes with all of them by default. I used the very minimum setup:

javamelody:
  # Enable JavaMelody auto-configuration (optional, default: true)
  enabled: true
  # Data source names to exclude from monitoring (optional, comma-separated)
  excluded-datasources:
  # Enable monitoring of Spring services and controllers (optional, default: true)
  spring-monitoring-enabled: true
  # Initialization parameters for JavaMelody (optional)
  # See: <https://github.com/javamelody/javamelody/wiki/UserGuide#6-optional-parameters>
  init-parameters:
    # log http requests:
    log: true
    # to exclude images, css, fonts and js urls from the monitoring:
    #url-exclude-pattern: (/webjars/.*|/css/.*|/images/.*|/fonts/.*|/js/.*)
    # to aggregate digits in http requests:
    #http-transform-pattern: \d+
    # to add basic auth:
    #authorized-users: admin:pwd
    # to change the default storage directory:
    #storage-directory: /opt/prod/xxxx/javamelody
    # to change the default "/monitoring" path:
    #monitoring-path: /admin/performance

We are also using melody in a Grails 5 and now Grails 6 and also just use in the same way as @gaolei, @Allison Martin the only change is that on each class we add the @MonitoredWithSpring, we found it fine use the latest 1.95.0 version

import net.bull.javamelody.MonitoredWithSpring


@Slf4j
@MonitoredWithSpring
class BaseController {

Your comments were helpful. We got things to work but plan on dropping the plugin and following the recommended alternate implementation as mentioned.