This used to be working fine in previous versions. I have been experiencing it now in 3.5.4 Pro. I'm on Windows 10 64-bit (32 GB RAM 1800 MHz DDR3). I tried with empty project and created new Instrument Track and set its output to some External MIDI Device. I have tried at least 3 different MIDI interfaces (iConnectivity mio10, ESI M4U XL and Yamaha UX-256). Non-emulated MIDI Outs don't work (although I can see MIDI events sent in the MIDI Monitor). With emulated ports it's really weird - when I play a MIDI sequence they work until I pause/stop playback. When I play again - nothing on the output (though - again - I can see MIDI activity on the internal SO MIDI Monitor, but nothing on the hardware). Restarting SO helps.