Ah yes, it’s true that some (most?) keyboards send a note_on with velocity 0 instead of a note_off.
In that case I think that probably the midi message is not triggering the sync command for some reason, so the synth command never gets called.
What about it you change the sync to: sync "/midi*note_on"?
There’s another thread here that looks similar and might give some clues: Midi in events not triggering