Major WD04 Section 3.14 Line 1833 says "If the Session State Expiry interval in the CONNECT packet was zero it is a protocol error to set a non zero Session Expiry in the DISCONNECT packet"
The text does not state how this error should be handled by the recipient. I don't think it would hurt to say 'The erroneous value will be treated as a zero, and the processing of the DISCONNECT message will proceed as described in section 3.14 (which says it MUST close the network connection and not send any more control packets, and not trigger a WILL message if the DISCONNECT return code is less than 128).
I still do not see the point in this and it just seems like special processing for DISCONNECT. There are quite a number of errors which can happen in common code while parsing DISCONNECT most of which will fall into some category of malformed packet or protocol error. If the receiver responds to these by sending a DISCONNECT and closing the connection it would be consistent with all other packets. I understand that this will create a race condition as the sender is probably about to close the network connection itself. Do we care which of the two completes first and closes the connection? There are a number of cases where we have similar race conditions to close a connection.
Issue included in MQTTv5.0 CS01 December 25, 2017
Fixed in WD10