CompletionStage documentation says:

if a stage's computation terminates abruptly with an (unchecked) exception or error, then all dependent stages requiring its completion complete exceptionally as well, with a

Unknown macro: {@link CompletionException}

holding the exception as its cause.

On the other hand, the CDI spec suggests that if an observer notification fails, the CompletionStage fails with FireAsyncException directly (not wrapped within CompletionException). This creates a conflict as the CDI spec uses the CompletionStage API without fully conforming to its contract.