On jdk17, following testcase works fine on Linux platform.

Testcase
==========
$ cat cstest1.java
import java.nio.charset.*;

public class cstest1 {
  public static void main(String[] args) throws Exception {
    Charset cs = Charset.defaultCharset();
    System.out.println(cs + ", " + cs.getClass() + ", " + cs.getClass().getModule());
  }
}
$ ~/jdk-17.0.6+10/bin/java -Dfile.encoding=Cp943C -showversion cstest1
openjdk version "17.0.6" 2023-01-17
OpenJDK Runtime Environment Temurin-17.0.6+10 (build 17.0.6+10)
OpenJDK 64-Bit Server VM Temurin-17.0.6+10 (build 17.0.6+10, mixed mode, sharing)
x-IBM943C, class sun.nio.cs.ext.IBM943C, module jdk.charsets
==========

But it does not work as expected on jdk18 and jdk21b06
==========
$ ~/jdk-18.0.2.1+1/bin/java -Dfile.encoding=Cp943C -showversion cstest1
openjdk version "18.0.2.1" 2022-08-18
OpenJDK Runtime Environment Temurin-18.0.2.1+1 (build 18.0.2.1+1)
OpenJDK 64-Bit Server VM Temurin-18.0.2.1+1 (build 18.0.2.1+1, mixed mode, sharing)
UTF-8, class sun.nio.cs.UTF_8, module java.base
$ ~/jdk-21/bin/java -Dfile.encoding=Cp943C -showversion cstest1
openjdk version "21-ea" 2023-09-19
OpenJDK Runtime Environment (build 21-ea+6-365)
OpenJDK 64-Bit Server VM (build 21-ea+6-365, mixed mode, sharing)
UTF-8, class sun.nio.cs.UTF_8, module java.base
==========