Raising initially as an edits issue following today's TC call. Ken believes this might have been an edits error and will check back through the change history.
Line 1740 currently says:
1740 A Server MUST send the Payload Format Indicator unaltered to all subscribers receiving the publication
1741 [MQTT-3.3.2-4]. The receiver MUST validate that the Payload is of the format indicated, and if it is not
1742 send a PUBACK, PUBREC, or DISCONNECT with Return Code of 0x99 (Payload format invalid) as
1743 described in section 4.13 [MQTT-3.3.2-5].
This is really restrictive as each message published will be checked at least twice, once by the server and once by the end receiver. We discussed this quite a bit on TC calls I thought the proposal was to make it a MAY clause.