Okay, I guess nobody will support us for that case because it is due to the way Play work in dev mode.
When staging the whole system, everything works fine. So the issue is most probably due to the fact the dev mode allow hot reloading of code changes. I guess it play with different classpaths or fork the JVM. Which explain why the key is found in some case but not in others.
I’m going to discuss that issue with Playframework.