I have a custom mesh that belongs to a dynamic batch group. I modify the vertices positions on each frame, while the index buffer remains the same. Do I need to mark the batch group as dirty on positions update?
yes, those vertices need to be copied to the new combined mesh.
but if you modify the mesh each frame, you could consider excluding it from the batch group, as the cost of recreating the new batch mesh could be higher than rendering the mesh by itself.