The code at hudson.plugins.groovy.Groovy.DescriptorImpl#setInstallations() does not implement the @CopyOnWrite semantics, see https://github.com/jenkinsci/groovy-plugin/blob/groovy-1.24/src/main/java/hudson/plugins/groovy/Groovy.java#L192

Instead of assigning the list atomically, an empty list is assigned and then filled.