same outboard synth?
To perform a loop back test you freeze a VST You can see I used SI drums and a Kick
You run the audio interface output back to an input and re record the sound. Make sure input echo is off on the new audio track.
Your use of an outboard synth would point a midi driver issue. Not anything to do with your interface unless your using it's midi ports.
Offset timing is hard to wrap your brain around.
Your input will be delayed comming in buy the input latency. So the DAW has to play the already recorded tracks ,including midi ahead of time to make up for that.
Your situation is your bypassing your audio output delay and the note is triggered sooner than it is supposed to be played. I'm only guessing. Is the connection to the outboard synth USB or MIDI?