If that doesn’t work, there is a buried setting in the matrixbrute, that chooses the source for sync. It is possible this got switched. Hook up to a PC and run artiria’s Midi Control Center software, or if you have firmware 2.0 installed on the synth, press preset+seq+mod to get to the settings page. Sync source is row D, the options are internal, clock (the analog sync 1/8” jack on the back), USB, or MIDI in. For the setup above, with 5 pin midi cable, you want the 4th button (MIDI) on row D switched on. You’d need to switch it to USB if you switched over to syncing the synth to a PC, via USB cable.
Good luck!