Custom gradle plugin throws Unable to load class 'io.github.classgraph.ClassGraph'

I’m using the ClassGraph library inside a custom gradle plugin: https://github.com/RoRoche/plantuml-gradle-plugin

It’s OK during unit testing and while releasing into local Maven and used in another project.

But since it has been released to gradle plugin portal (https://plugins.gradle.org/plugin/com.github.roroche.plantuml), when using inside another project (https://github.com/RoRoche/eo-plantuml-builder), when running my buildClassDiagram task, it throws

* What went wrong:
Execution failed for task ':buildClassDiagram'.
> Lio/github/classgraph/ClassGraph;

Any idea of what’s going wrong? Gradle plugin packaging?