I made a simple tessellated material based solely on Perlin noise.
In many configurations the “Crack Free Displacement” option in tessellation setting caused polygon edges to cause weird displacement results / seams / hard cuts.
While typing this question I finally found the problem in my material. I still posted it so it might help others.
I had a “Transform Vector” node that transformed the noise from world to tangent space. This caused the issue. The node was a leftover from a tutorial.