You have NOT described a standalone machine, it can’t be if it’s connected by USB

There are a number of things which can result in this MIDI issue and it all depends on what you have set up at the other end of the USB cable … as you are not too familiar with the MIDI side i’d suggest going to the Menu item in AR which deals with PAD MIDI note destination, switch that from Int + Ext to just Int !

That will stop the issue, but then you have to explore MIDI and your machine settings and contemplate the possibilities