The minimal code change to workaround the issue is to chage the outer class method to package local access.
Jars created with OpenJDK 11.0.4 on Windows 10. TestIn.jar: executable jar file with original class file TestOut.jar: executable jar file created with ProGuard 6.1.1
ProGuard 6.1.1 creates invalid byte code for nest based access (Java 11)