I have been using the same recipe for years and all of a suddrn the cookies spread while baking to a very soft and loose shape rather than a crisp outline.
I use 3 1/2 cups flour, 1/2 cup rice flour, 1 teaspbaking soda and a bit of salt, 1 lb. butter and a quarter inch slice off a pound of lard. I have tried both 300 and 350 degrees in the oven and they all turn out the same.
Is your baking soda fresh?
Have you changed brands of butter?
That's all I can think of, I don't think it would be an oven temp thing but check the oven temp with a thermometer.
(and sugar? )
Is the flavor OK, just the spreading is an issue?
are you using a cool baking sheet? a warm one (between batches) will start to melt the fat before the flour starts to cook.
If this is shortbread, why not bake in a rimmed pan and cut into traditional shortbread squares?
Cookie spread is almost always related in some way to the butter not being chilled enough prior to baking. Have you changed the manner in which you soften the butter? Are you chilling the dough/formed cookies any differently?
If you're not chilling the formed/cookie sheeted cookies, do.