I'm not sure if this is a driver issue or a breaking change with Atlas.

I only see the issue when I connect to an Atlas server, if I connect to a 3.12 replicaset server (on my local network) then the code example works fine.

I'm using the free version of Atlas in a 3 node replicaset configuration (default?). Nothing else configured.

var client = new MongoClient(...);
var database = client.GetDatabase(MongoUrl.Create(connectionString).DatabaseName);

var userAccounts = }}{{database.GetCollection<BsonDocument("userAccounts")
{{ .WithWriteConcern(WriteConcern.Acknowledged);}}

var logging =
{{ database}}.GetCollection<BsonDocument>("logging").WithWriteConcern(WriteConcern.Unacknowledged);

userAccounts.InsertOne(new BsonDocument());

await logging.InsertOneAsync(new BsonDocument());

{{userAccounts.InsertOne(new BsonDocument()); // <-- deadlock here }}

If I change the logging collection to be WriteConcern.Acknowledged then the deadlock goes away.

Async or Sync methods don't make any difference.