It’s the midi program change message. I can’t remember all the details but it’s possible to stop sending those by going into the midi section of the configuration. You’ll find that program change is abbreviated PCH on the M:S menus.
Config -> MIDI -> Sync -> PCH Out - looks like this will do what you want if you set it to off
You might also mess with:
Config -> MIDI -> Ports -> Out to - this can be set to off which switches off all midi … may turn off your sync though, try it.
Config -> MIDI -> Out Chan - can be set to off, but you’ll need to switch off every channel, and I’m not clear if that includes the notes only, or notes + PCH