Compiler version

Scala 3.0.0, OpenJDK 15.0.2, sbt 1.5.2,

Minimized code

TestenumS.scala

object TestenumS :
    def go() = println("Scala: Testme Hello= " + Testme.Hello)

enum Testme extends java.lang.Enum[Testme] :
    case Hello

TestenumJ.java

public class TestenumJ {
    public static void main(String[] args) {
    TestenumS.go();    // line 3 calls Scala to look at Testme
    System.out.println("Java: Testme Hello= " + Testme.Hello); //=> null, unless line 3 commented, then => Hello
    }
}

Output

[info] running TestenumJ 
Scala: Testme Hello= Hello
Java: Testme Hello= null

Expectation

Java: Testme Hello= Hello (not null !!)

It works as expected (Hello) only when line 3 is commented, i.e. when java sees the enum before scala.
As it compiles ok, so the null propagates to cause runtime errors elsewhere, it can takes a long time to find the cause
(especially when it looks ok called from scala)