12.15 SubscriptionDiagnosticsDataType
Table 238 – SubscriptionDiagnosticsDataType Structure
monitoringQueueOverflowCount
Description:
The number of times a monitored item dropped notifications because of a queue overflow.

In Part 4 we define (was clarified a while ago):
5.12.1.5 Queue parameters
If a Notification is discarded for a DataValue and the size of the queue is larger than one, then the Overflow bit (flag) in the InfoBits portion of the DataValue statusCode is set.

In Part 4, the overflow is only indicated if the queue size is large than one.
It is not clear if this is also the case for Part 5 monitoringQueueOverflowCount.

I think this should be consistent but needs to be clarified.