Making algebraic surfaces in my MathView, I can’t merge vertices with BufferGeometryUtils.mergeVertices(). Increasing tolerance gives nothing, but large holes in the surface. Here is an example:
Maybe your boundary vertices are too different and cannot be merged?
Merging compares not only positions, but also normals and uvs. So if the uv’s at both sides of the seam are different, then the vertices will not be merged even if their positions are identical.
One way to merge it to ensure all data is the same (especially the uvs). Another way is to delete the normals and uvs, merge, and then recreate them.