Seemed fine to begin with then, after changing code repository I worked on, I noticed that some JUnit tests did not get the "Play" decorations. Indeed the TestExplorer was empty. Then after "rebuild" ...

Write a Java test class with syntax errors (Copilot may do this). Nudge Copilot to run the test using the native test runner (not gradle or maven). VSCode will prompt in the UI with a compile error, ...