So I meant to submit this back in 1.1.1 or so. Left the image in the wrong file and completely forgot about it.
Void Door. Game seemed to have forgot to apply the ground tile texture for randomly spawned ruin doors out in the forest.
This particular ruin is right next to the entrance of Flooded Cavern. By that I mean on the same map as the entrance and literally only several spaces away. If that added information means anything or helps you locate the issue.