Easiest said in pictures. I can probably cut it right down to the troubling corner if you need a blend (It’s part of a much much larger scene). It’s the only place it happens.
It’s a single, much larger face with a subdivision surface modifier, and the texture is entire procedural.
And for good measure, the UV is clean (I’ve also checked connected points)
It’s the only place on the model it occurs, and with subd turned off, it goes away. Not using adaptive subd (though I tried, and it makes no difference).
EDIT: After further testing, the higher the subdivision the worse it gets, but only evident with Catmull-Clarke. Simple subd doesn’t do it.