(based on conversation over here openrewrite/rewrite-gradle-plugin#305)
In order to customize plugin marker artifacts, the work must be done in an afterEvaluate block, as described here:
https://discuss.gradle.org/t/how-can-i-customize-the-pom-of-the-plugin-marker-artifacts/33053
The MavenApacheLicensePlugin does not do this. So, a project using this plugin and publishing plugins to Gradle Plugin Portal will successfully have the license in the plugin jar's generated pom, but, the license will not be present in the plugin marker artifacts.