Did your problem just go away by its self?
As far as I can tell, though I have also restarted the game since noticing the problem. I didn't stop to play around with it very much, but iirc when I first tried to trade with Arron in the grove when using the Trade view. I think I then bought some dye from Mattis and used the Barter view, and didn't have the problem.
I don't think I've probably tried to purchase a stackable item since, until I checked just now with Arron again and it seemed fine, prompting me to select the quantity in both the Trade and Barter views. There's clearly something buggy here, but in advance of a proper fix I'll hope the issue resolves itself for you too!