ADDITIONAL SYSTEM INFORMATION :
tested on linux and windows

A DESCRIPTION OF THE PROBLEM :
java.io.File.createTempFile("xyz", ".tmp") uses default temp-dir event if 
System.setProperty("java.io.tmpdir", ...); was set before.

It seems that this was changed between java 19 and 20

One possible change was in java.nio.file.TempFileHelper
from
private static final Path tmpdir = Paths.get(doPrivileged(new GetPropertyAction("java.io.tmpdir")));
to
private static final Path tmpdir = Path.of(StaticProperty.javaIoTmpDir());

StaticProperty is loaded very very early and therefore jdk.internal.util.StaticProperty.JAVA_IO_TMPDIR contains the default temp-dir.
-> I know that java.io.tmpdir has to be set early in application but currently even a simple main-method is not able to change the java.io.tmpdir (see steps to reproduce)

REGRESSION : Last worked in version 19

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
call System.setProperty("java.io.tmpdir", ...) on application start, create temp-file and check the directory.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
no exception 
ACTUAL -
exception beginning with java 20

docker run -it --rm -v $PWD:/test -w /test eclipse-temurin:11-jdk sh -c "javac Test.java && java -cp . Test"
11.0.21
/tmpfoobar
/tmpfoobar/xyz12142257841535750632.tmp

docker run -it --rm -v $PWD:/test -w /test eclipse-temurin:17-jdk sh -c "javac Test.java && java -cp . Test"
17.0.9
/tmpfoobar
/tmpfoobar/xyz3659998787174078425.tmp

docker run -it --rm -v $PWD:/test -w /test eclipse-temurin:19-jdk sh -c "javac Test.java && java -cp . Test"
19.0.2
/tmpfoobar
/tmpfoobar/xyz16800814432785594764.tmp

docker run -it --rm -v $PWD:/test -w /test eclipse-temurin:20-jdk sh -c "javac Test.java && java -cp . Test"
20.0.2
/tmpfoobar
/tmp/xyz7255378145959570555.tmp
Exception in thread "main" java.lang.IllegalStateException: actual: /tmp, expected: /tmpfoobar
        at Test.main(Test.java:14)

docker run -it --rm -v $PWD:/test -w /test eclipse-temurin:21-jdk sh -c "javac Test.java && java -cp . Test"
21.0.1
/tmpfoobar
/tmp/xyz1465904699981539377.tmp
Exception in thread "main" java.lang.IllegalStateException: actual: /tmp, expected: /tmpfoobar
        at Test.main(Test.java:14)


---------- BEGIN SOURCE ----------
public class Test {
  public static void main(String[] args)
   throws Exception
  {
    final java.io.File tmp = new java.io.File("/tmpfoobar");
    tmp.mkdirs();
    System.setProperty("java.io.tmpdir", tmp.getAbsolutePath());
    java.io.File testFile = java.io.File.createTempFile("xyz", ".tmp");
    System.out.println(System.getProperty("java.version"));
    System.out.println(tmp.getAbsolutePath());
    System.out.println(testFile);

    if (!tmp.getAbsolutePath().equals(testFile.getParent()))
      throw new IllegalStateException("actual: " + testFile.getParent() + ", expected: " + tmp.getAbsolutePath());
  }
}

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

FREQUENCY : always