SseEventOutputImpl wrong CompletionStage composition sequence

CompletionStage<Void> a = internalFlushResponseToClient(true);
         CompletionStage<Void> b = writeEvent(event);
         return a.thenCompose(v -> b);

return internalFlushResponseToClient(true)
        .thenCompose(v ->  writeEvent(event));

CompletionStage<Void> a = aos.asyncWrite(SseConstants.DOUBLE_EOL);
 CompletionStage<Void> b = aos.asyncFlush();

CompletionStage<Void> a = aos.asyncWrite(SseConstants.DOUBLE_EOL)
 .thenCompose(v -> aos.asyncFlush());

CompletionStage<Void> a = aos.asyncWrite(bout.toByteArray());
 CompletionStage<Void> b = aos.asyncFlush();

return aos.asyncWrite(bout.toByteArray())
        .thenCompose(v ->  aos.asyncFlush())
......