if you call gtk_tree_view_expand_to_path (view, path), 
gtk_tree_view_row_expanded (view, path) returns FALSE.

It means that gtk_tree_view_expand_to_path() doesn't work or has wrong
documentation.

Test case.
----------

#include <string.h>
#include <stdlib.h>
#include <stdio.h>

struct ListSort
{
  gint number1;
  gint number2;
};

struct ListSort data[] =
{
  { 3, 4 },
  { 2, 6 },
  { 0, 49 },
  { 856856, 965 },
  { 1, 34663 },
};

struct ListSort child_data[] =
{
  { 10, 149 },
  { 12, 13 },
  { 1856856, 1965 },
  { 11, 15 },
  { 14, 14 },
};

const gint NUM_TREE_ELEMS = 5;

int
main (int argc, char** argv)
{
    GtkTreeStore* store;
    GtkTreeViewColumn* column0;
    GtkTreeViewColumn* column1;
    GtkCellRenderer* renderer;
    GtkWidget* tree_view;
    GtkTreeIter iter;
    GtkTreeIter child_iter;
    gint i;
    gint j;

    gtk_init (&argc, &argv);

    store = gtk_tree_store_new (2, G_TYPE_UINT, G_TYPE_UINT);

    for (i = 0; i < NUM_TREE_ELEMS; i++)
    {
        gint j;
        
        gtk_tree_store_append (store, &iter, NULL);
        gtk_tree_store_set (store, &iter,
                            0, data[i].number1,
                            1, data[i].number2,
                            -1);
        for (j = 0; j < NUM_TREE_ELEMS; j++)
        {
            gtk_tree_store_append (store, &child_iter, &iter);
            gtk_tree_store_set (store, &child_iter,
                                0, child_data[j].number1,
                                1, child_data[j].number2,
                                -1);
        }
    }
    
    tree_view = gtk_tree_view_new_with_model (GTK_TREE_MODEL (store));
    
    renderer = gtk_cell_renderer_text_new ();
    column0 = gtk_tree_view_column_new_with_attributes ("Number 1",
renderer,
                                                        "text", 0,
                                                        NULL);
    gtk_tree_view_append_column (GTK_TREE_VIEW (tree_view), column0);
    gtk_tree_view_column_set_sort_column_id (column0, 0);
    
    renderer = gtk_cell_renderer_text_new ();
    column1 = gtk_tree_view_column_new_with_attributes ("Number 2",
renderer,
                                                        "text", 1,
                                                        NULL);
    gtk_tree_view_append_column (GTK_TREE_VIEW (tree_view), column1);
    gtk_tree_view_column_set_sort_column_id (column1, 1);
    

    {
        GtkWidget* window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
        gtk_container_add (GTK_CONTAINER (window), tree_view);
        gtk_widget_show_all (window);
    }

    {
        const char* str = "2:2";
        GtkTreePath*  expand_to_path = gtk_tree_path_new_from_string (str);
        gtk_tree_view_expand_to_path (GTK_TREE_VIEW (tree_view),
expand_to_path);
        printf ("--- path = '%s', expanded = %d\n",
                str,
                gtk_tree_view_row_expanded (GTK_TREE_VIEW (tree_view),
expand_to_path));
    }

    gtk_main();
}

Output.
-------
--- path = '2:2', expanded = 0