It seems something is wrong with the way the subscription model in 5.14.1 describes when to increment a sequence number.

Given the following exchange with UaCPP Demo server:

(initial notification)
2025-04-02 13:46:18:083 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=1, expectedSequenceNumber=1
2025-04-02 13:46:18:084 subscription onDataReceived: nodeId=NodeId{ns=3, id=Demo.Static.Scalar.Int16}, value=Variant{value=0}

(keep-alive notifications)
2025-04-02 13:46:28:175 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=2, expectedSequenceNumber=2
2025-04-02 13:46:38:239 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=2, expectedSequenceNumber=2
2025-04-02 13:46:48:334 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=2, expectedSequenceNumber=2

(write a new value, expect a change)
2025-04-02 13:46:52:355 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=2, expectedSequenceNumber=2
2025-04-02 13:46:52:356 subscription onDataReceived: nodeId=NodeId{ns=3, id=Demo.Static.Scalar.Int16}, value=Variant{value=1}

(keep-alive notifications)
2025-04-02 13:47:02:426 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=3, expectedSequenceNumber=3
2025-04-02 13:47:12:484 Received PublishResponse, subscriptionId=3999684478, sequenceNumber=3, expectedSequenceNumber=3

according to the spec, shouldn't the sequenceNumber of the PublishResponse with the value change have been incremented?

Once the value changed, the Subscription would have been in State #14 in the Subscription State Table. The actions:

ResetLifetimeCounter()
StartPublishingTimer()
DequeuePublishReq()
ReturnNotifications()
MessageSent = TRUE

ReturnNotifications() is defined as:

CreateNotificationMsg()
ReturnResponse()
If (MoreNotifications == TRUE) && (PublishingReqQueued == TRUE)
{
    DequeuePublishReq()
    Loop through this function again
}

The first line of CreateNotificationMsg() calls for the sequence number to be incremented:

Increment the SeqNum and create a NotificationMessage from the MonitoredItems assigned to the Subscription.
Save the newly-created NotificationMessage in the retransmission queue.
If all available Notifications can be sent in the Publish response, the MoreNotifications state Variable is set to FALSE. Otherwise, it is set to TRUE.

So why doesn't it increment?