Method $proxy$xxx has illegal modifiers: 0x1008

ADDITIONAL SYSTEM INFORMATION :
Ubuntu Linux 20.04, OpenJDK Runtime Environment (build 19-ea+25-1892)

A DESCRIPTION OF THE PROBLEM :
I have a sealed interface implemented by records and try pattern matching on the record types (JEP 405). The program fails to run and displays the message "Method $proxy$seq in class SubSequence has illegal modifiers: 0x1008"

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run the source file below.



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The program runs and prints "sip"
ACTUAL -
The program dies with the message

Method $proxy$seq in class SubSequence has illegal modifiers: 0x1008

---------- BEGIN SOURCE ----------
import java.util.*;

sealed interface SubSequence extends CharSequence {
   CharSequence seq();

   default int start() {
      return 0;
   }

   default int end() {
      return seq().length();
   }

   default char charAt(int index) {
      Objects.checkIndex(index, length());
      return seq().charAt(start() + index);
   }

   default int length() {
      return end() - start();
   }
   
   default CharSequence subSequence(int s, int e) {
      return new SubSequence.Middle(seq(), start() + s, start() + e);
   }

   record Initial(CharSequence seq, int end) implements SubSequence {
      public Initial {
         Objects.checkIndex(end, seq.length());
      }
      public String toString() {
         return seq.subSequence(0, end).toString();
      }
   }
   record Final(CharSequence seq, int start) implements SubSequence {
      public Final {
         Objects.checkIndex(start, seq.length());
      }
      public String toString() {
         return seq.subSequence(start, seq.length()).toString();
      }
   }
   record Middle(CharSequence seq, int start, int end) implements SubSequence {
      public Middle {
         Objects.checkFromToIndex(start, end, seq.length());
      }
      public String toString() {
         return seq().subSequence(start, end).toString();
      }
   }
   static CharSequence simplify(SubSequence seq) {
      return switch (seq) {
      case Initial(var cs, var e) when e == cs.length() -> cs;
      case Final(var cs, var s) when s == 0 -> cs;
      case Middle(var cs, var s, var e) when s == 0 && e == cs.length() -> cs;
      default -> seq;
      };
   }
}

public class Test2 {
   public static void main(String[] args) {
      System.out.println(SubSequence.simplify(new SubSequence.Final(
            new SubSequence.Initial("Mississippi", 6), 3)));
   }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Don't use pattern matching

FREQUENCY : always