In function start_workers,workers fork VLIB_NODE_TYPE_INTERNAL nodes from vlib_thread_main:

nm_clone->nodes_by_type[VLIB_NODE_TYPE_INTERNAL] =
vec_dup (nm->nodes_by_type[VLIB_NODE_TYPE_INTERNAL]);
vec_foreach (rt, nm_clone->nodes_by_type[VLIB_NODE_TYPE_INPUT])
{
vlib_node_t *n = vlib_get_node (vm, rt->node_index);
rt->thread_index = vm_clone->thread_index;
/* copy initial runtime_data from node */
if (n->runtime_data && n->runtime_data_bytes > 0)
clib_memcpy (rt->runtime_data, n->runtime_data,
clib_min (VLIB_NODE_RUNTIME_DATA_SIZE,
n->runtime_data_bytes));
}

nm_clone->nodes_by_type[VLIB_NODE_TYPE_INPUT] should be nm_clone->nodes_by_type[VLIB_NODE_TYPE_INTERNAL].