The MIDI cable was defect 😯
However the DDX3216 only transmits MMC when SOURCE (AUTOMATION) is set to other than CLOCK.
Of course this isn't logic at all. But there's more, the DDX3216 does not transmit anything out when SOURCE is set to CLOCK. So I wonder how any device can be coupled for sync purposes when the DDX3216 MMC and AUTOMATION settings contradict this.