Async (await) version IMongoCollection.UpdateOneAsync occasionally fails

With this class:

class Block {
    ObjectId Id { get; set; }
    List<Item> ListOfItems;
}

I'm testing the following method, that just adds and Item to certain Block's ListOfItems field:

async Task AddItem(Block block, Item item) {
    var updateDefinition = Builders<Block>.Update
        .AddToSet(b => b.ListOfItems, item);

    var result = await blocksCollection.UpdateOneAsync(
        b => b.BlockId == block.BlockId, updateDefinition);

    // however the synchronous version works:
    // var result = blocksCollection.UpdateOneAsync(
    //      b => b.BlockId == block.BlockId, updateDefinition).Result;

    if (result.ModifiedCount != 1)
        throw new DbException();
}

This is the test:

async Task AddItemsTest() {
    await Task.WhenAll(Enumerable.Range(0, 20).Select(
        i => AddItem(certainBlock, new Item())));
}

Now if I run it asynchronously (with await on blocksCollection.UpdateOneAsync method), sometimes the result.ModifiedCount is 0 and not 1 (approx. 1 to 17 times), which throws the DbException.

When I run it synchronously (no await, with .Result), everything runs fine and all 20 items are added.

Why is that? what's wrong with the async version?