OPC UA Subscription in a C# Client app doesn't trigger event handlers|OPC UA Implementation: Stacks, Tools, and Samples|Forum|OPC Foundation

I have a FactoryTalk server with some nodes and I need to monitor several attribute values.

I made a client which reads and writes data succesfully, but I can't get the subscription to work.

Here's the subscription configuration:

subscription = new Subscription(session.DefaultSubscription)
{
DisplayName = "Test subscription",
PublishingEnabled = true,
PublishingInterval = 500,
LifetimeCount = 0,
};

My monitored items:

monitoredItems = nodeAttrs
.Select(nodeId => new MonitoredItem(subscription.DefaultItem)
{
StartNodeId = nodeId,
AttributeId = Attributes.Value,
QueueSize = 1,
SamplingInterval = 500,
MonitoringMode = MonitoringMode.Reporting,
})
.ToList();

And I assign them the notification handler (I need the protocol object instance here, so I tweaked it):

MonitoredItemNotificationEventHandler monitoredItemEventHandler = (monitoredItem, e) => AttributeNotification(monitoredItem, e, protocol);

Then I add the event handlers:

foreach (var monitoredItem in monitoredItems)
{
monitoredItem.Notification += monitoredItemEventHandler;
}

And I add the items to the subscription and create it:

subscription.AddItems(monitoredItems);
session.AddSubscription(subscription);
subscription.Create();

I've checked through the console and the subscription gets created and assigned an ID by the server. I check all the monitored items as well, CreateMonitoredItems service call succeeds, and they also get client handles assigned, NodeId's are correctly resolved, with no error StatusCodes.

I make changes to the attributes via a 3rd party client application called UaExpert.

Yet I can't get the notification event handler to fire off.

Did I do something wrong?
Does anyone have advice on how to resolve/debug this?