If you're able to assign None as Input to MIDI/simple Instrument tracks, and it stays that way, that implies you're not using the
Always Echo Current MIDI Track setting in Preferences to echo your controller input. If that's the case, only tracks with the Input Echo button manually enabled about echo the input. Double-check you have echo enabled on only the MIDI track pointing to the synth you want to hear. Also, if the Motif is involved, are you monitoring it through an audio track in SONAR and have you set Local Control Off?
If that doesn't explain it, please provide a little more detail on what tracks/synths you have in the project, and how things are routed/monitored.