I unlocked the SotT portrait “Priestess of the Thorn” for completing all the SotT achievements, and it seems like the achievement portrait and the free portrait, “Queen of Thorns” should be swapped.
The text for “Priestess of the Thorn” speaks of beginning a journey and the art for the portrait is simple and muted. The text for Queen of Thorns tells of a journey completed and the art is much more ornate.
It seems strange that “Queen of Thorns” with its flashier art and text about completing a journey is the free portrait while “Priestess of Thorns” is the reward for completing all SotT achievements. I’m not sure this is a bug exactly, but it does seem like an implementation oversight at least.