On the build page of Maven/Freestyle Jobs, old 3D ball icons are used for the Subproject Builds:

These are green with the deprecated Green Balls plugin and blue without it.
Instead, the modern flat icons of Jenkins should be used.

In this pull request it was pointed out that the responsible code line seems to be https://github.com/jenkinsci/parameterized-trigger-plugin/blob/f6343e3e2288ffb4e889fb4f8bb317d9f5696084/src/main/resources/hudson/plugins/parameterizedtrigger/BuildInfoExporterAction/summary.groovy#L21.