Pipeline with async hangs

[vksnk]

Following pipeline never completes:

        Func producer1, producer2, consumer;
        Var x, y;

        producer1(x, y) = x + y;
        producer2(x, y) = producer1(x, y);
        consumer(x, y) = producer1(x, y - 1) + producer2(x, y + 1);

        consumer.compute_root();

        producer1.compute_at(consumer, y).async();
        producer2.store_root().compute_at(consumer, y).async();

        consumer.bound(x, 0, 16).bound(y, 0, 16);

        Buffer<int> out = consumer.realize(16, 16);

[vksnk]

It works correctly if producer1 is also scheduled as store_root().