One of Miroslav's comments:

Like the CONNECT packet, this PINGREQ packet contains the Client Id.
What is the purpose of the Client Id in this packet? The gateway must be able to identify the client using its network address anyway.

What should gateway do if the given Client Id was different than the Client Id given by the previous CONNECT? If it should update the session, it would also have to re-authenticate the client. And a transparent gateway should reestablish the MQTT connection. This is problematic (see 1.2).