Hi there, 

I think it would be useful for us to see the settings / configuration on your attachment controls. I tried to replicate your scenario - added two attachment controls in classic, but, I can't get the same behavior. How many attachments are we talking about here?

@JasonForbes 

You have all your files in the same control because the control is the default control for attachments. If you upload files outside of the Nintex form, this default attachment control will display all files that were added outside of the Nintex form. This is the behavior you are seeing.

And this behavior makes me ask 2 remarks/questions :

-First of all, why have JS on the form? what exactly does this JS do, to add the PCs to an item the native Nintex controls are sufficient. Can you tell us what this JS does ? if it is the JS who manages all the Attachments part, then it is normal that you have this behavior, because you do not use the native operation of nintex.

-Secondly, is it the same list form (and same SharePoint item) that you are looking at?

Example: You create an item by attaching attachments to it in a list A, and you duplicate the item in a list B, you display this item from list B.