I have no idea what it could be. The OP says, beyond all doubt, the only difference between the 2 beers is the Rye. So, I guess it’s the Rye. I’m not dying on this hill. ;D
Not that this has anything to do with the OP and the Scottish Ale, but interestingly, Brulosophy did an experiment that is on-point to this conversation. And they did find a difference between a beer made with Rye and one with not. No mention of increased bitterness, but reading between the lines, I can see where one can get “spicy” from Rye.

exBEERiment | Impact Rye Malt Has On American Pale Ale
Author: Matt Del Fiacco A wide variety of cereal grains in many different forms are available to brewers for use in the production of beer. One such grain that is seeing an increase in popularity, …