As we have already communicated to the user, the issue in question is not a bug.
When merging the meshes, the positions of the vertices shift slightly, which in turn affects the results of the Point Light shadow calculations.
Since the vertex positions are only altered minimally, the mesh appears visually identical before and after the merge.
However, because the Point Light is positioned very close to the mesh, even a tiny difference can result in significant variations in shadow calculations.