1968 3.4.2.4 Reason String
1969 38 (0x26) Byte, followed by UTF-8 String Pair.
This should be 3.4.2.4 User Defined Property.
In addition, how are we selecting between:
MUST NOT use this Identifier/Value pair if it would increase the size of the PUBACK packet beyond the Maximum Packet Size specified by the Client
and
The sender must not send this property if it would increase the size of the PUBACK beyond the Maximum Packet Size specified by the session partner.