encountered this from this post, Changing synth velocity with variable - Audio - Unreal Engine Forums,
basically once you use the note on function of a synth, if you try to play another note with a different velocity the synth will keep playing the first notes velocity.
to recreate:
Create a blueprint with a synth component and synth preset variable. Enable inputs and have a keypress assigned to the synth’s “Note On” function. Enter an arbitrary note, I used 60, and a high velocity (100). Create another kepress function and have it play another note but with a low velocity (30, so you can still hear it).
Now what happens when you trigger one of the keypress it will be correct the first note, but then try the other keypress with the different velocity, it will have the same volume as the first keypress. Try it again but reverse the order and you should hear that the first note is correct and the second note played will match the first note’s velocity and not its’ own.
Maybe I am missing something but seems like a bug.