I think this is just the nature of long samples, if it’s one sample which lasts the entire length of the bar and then you’re trying to mute before it’s played itself to completion, you can use slice machine to cut it and then use the option to lay the slices on consecutive trigs, so then when you hit mute it will mute at the next slice essentially cutting the note off.
With a sample I don’t think you can achieve immediate note off via midi command the way you’re thinking. You can try it though.