Usually when cookies bend or bubble unevenly during baking, it's a sign of too much working (re-rolling) of the dough, and activation of the gluten in the dough - or the baking surface heating the cookies unevenly. The former (activating gluten) in shortbread is hard to do, since this dough has proportionally lots of sugar and butter in it, which tend to have a balancing (softening) effect. Have you noticed this more on the second roll of the dough. If so, it may be the gluten issue. Are you baking on silicone mats? You might try that, as they can even out heat distribution.
But, honestly, I don't think it's a major issue. It's the underside of the cookie, and I'm sure they still taste great and look perfect from the top.