In the scenario of a cable disconnection between the BMENUA0100 (OPC UA Server) and an OPC UA Client the number of client (CONNECTED_CLIENT) from the BMENUA0100 device DDT will not decrease instantly.
In this scenario, the BMENUA0100 is working as designed by the OPC UA specification, because state machine of the OPC UA is different if we compare for a cable disconnection and closing the OPC UA Client. A cable disconnection is a non graceful client close and in this case, the session of the client does not close until the session times out. The OPC UA server (BMENUA0100) is not supposed to close the session until the server is sure that the client is gone and the session timeout is the only mechanism available when a client does not shut down gracefully.

The OPC UA timeout is configured by the OPC UA client including the session timeout and the most common value is 360000 ms. In a non gracefully client disconnection, the OPC UA Server will close the session (decrease the number of client) only when it times out. As the connection still activated when the cable is unplugged, the OPC UA Server still doing same activities and the number of packets still high for a while.

P.S.: You can find here the OPC UA specification that includes the behavior seen by the customer.

https://reference.opcfoundation.org/v104/Core/docs/Part4/5.6.2/

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