I assume that you're using the MxB as a sync slave here, with "MIDI Sync" set to "On" (the "MIDI Sync" label is a bit misleading since it also syncs to the "Sync" input). When the MxB is a sync slave it will not transmit any sync itself. AFAIK, this is the same for most (all?) synths. If you want to sync several devices to one master, it's best to connect all slaves directly to the master (using MIDI through to daisy chain devices is also fine for this).
If you have a mixed MIDI clock and analog sync setup, you need a master capable of sending both. E.g. the MatrixBrute can do this.