Do you use seam persistence module as well?
You can try adding @Transactional annotation to that method.
I just switched to the seam persistence module, @injecting the entitymanager.
It works fine when not using @Asynchronous.
When using @Asynchronous, I get the following exception:
org.jboss.weld.context.ContextNotActiveException: WELD-001303 No active contexts for scope type javax.enterprise.context.ConversationScoped
The short answer is that only Application and Dependent scopes are supported for asynch and scheduled invocations, currently. There is an open JIRA for further investigation into what else is possible, to which I have added a comment relevant to this post.
Probably a good idea to watch that issue to keep in the loop.