javac fails to associate type annotations with TypeMirrors for some symbols loaded from the classpath, which prevents plugins from accessing those annotations across compilation boundaries. The annotations are present if the same symbol is compiled from source in the compilation where the plugin runs.

=== ./test/B.java
abstract class B extends A {}
=== ./test/A.java
import java.lang.annotation.ElementType;
import java.util.List;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE_USE)
@Retention(RetentionPolicy.RUNTIME)
@interface TypeAnnotation {}

abstract class A implements List<@TypeAnnotation String> {}
=== ./plugin/p/P.java
package p;

import com.sun.source.util.JavacTask;
import com.sun.source.util.Plugin;
import com.sun.source.util.TaskEvent;
import com.sun.source.util.TaskListener;
import javax.lang.model.element.TypeElement;
import javax.lang.model.type.DeclaredType;
import javax.lang.model.type.TypeMirror;

public class P implements Plugin {

  @Override
  public String getName() {
    return "P";
  }

  @Override
  public void init(JavacTask javacTask, String... strings) {
    javacTask.addTaskListener(
        new TaskListener() {
          @Override
          public void finished(TaskEvent e) {
            if (e.getKind() != TaskEvent.Kind.ENTER) {
              return;
            }
            TypeElement b = javacTask.getElements().getTypeElement("B");
            for (TypeMirror i :
                ((TypeElement) ((DeclaredType) b.getSuperclass()).asElement()).getInterfaces()) {
              System.err.printf("%s %s\n", i, i.getAnnotationMirrors());
            }
          }
        });
  }
}
=== ./plugin/module-info.java
module p { 
  requires transitive jdk.compiler; 
  provides com.sun.source.util.Plugin with p.P; 
} 
===

$ javac $(find plugin -name '*.java')

# when both compilation units are compiled from source, the type annotations are visible

$ javac --processor-module-path plugin -Xplugin:P test/A.java test/B.java 
java.util.List<@TypeAnnotation java.lang.String> 
java.util.List<@TypeAnnotation java.lang.String> 

# when 'A' is loaded from the classpath, the type annotations on its supertype are not visible

$ javac --processor-module-path plugin -Xplugin:P -classpath test test/B.java 
java.util.List<java.lang.String>