notes become sustained when a bunch of short notes play.

When a lot of short notes play, one or more notes play longer than they should, eventually being stopped when another note of the same pitch plays, if that makes sense. The part used in the video is at 1:27, another part that this can happen is at 3:00, and the F note that is sustained is actually an A but it is distorted because the sf2 editor i used distorts high notes, although this happens throughout the entire keyboard. I recommend using a soundfont that doesn't fade when played (an example of this is using a violin sf2 instead of a piano sf2).I hope this helps, thanks.