Have you tried upgrading the sonar-java plugin version that you rely on? In the repository you shared I see in the root pom.xml file that the <sonar.java.version> is set to 7.30.0.34429 which is quite old, and I think the issue that you have is related to the classpath being empty in the InternalCheckVerifier that comes from that version, which used a different way of loading the classpath.
Please update to some later sonar-java released version and let me know if the behavior changes