It doesn’t appear to be related to the blueprint, because I made an instance and applied it directly to the mesh, with the same result.
However, I noted that it gives this “peak and valley” effect, with the peaks being correct, and the valleys being incorrect. I just counted the peaks - exactly 40, the number of triangles in the mesh. It seems that the tessellation is not shifting any points along the original edges in the mesh - only between them (except on the border edges). Is this intended behaviour, and if so, can I do anything about it?
I have somehow managed to get the effect to be small enough to not be noticeable, for now. I would still like to know the cause, and how to fix it, if possible.