For the steps I describe, you have to have subdirectories in your home directory, and some of those subdirectories should also contain subdirectories.

When I was using Xubuntu 18.04 with Thunar 1.6.15, the tree view inside the side pane worked like this:
1. Focus a directory on the side pane tree view (say, my home directory).
2. Press the right arrow key to expand it.
3. I can go through its subdirectories (move the focus down along the tree) using the down arrow.
4. I press the right arrow key to expand the subdirectory I want to dive into.

*Steps to reproduce this bug:*

Now, after upgrading to Xubuntu 19.04 with Thunar 1.8.4, it works like this:
1. Focus a directory on the side pane tree view (my home directory).
2. Press the right arrow to expand it.
3. When I press the down arrow, it automatically expands the focused subdirectory (if it has subdirectories). To actually go down the tree view using the down arrow, I first have to close the just expanded subdirectory using the left arrow. Expected result is as above in steps 3-4.

This makes navigating directories really cumbersome, as I cannot just blindly use "down, left" either to actually go down: if a subdirectory focused with the down arrow key does not have subdirectories of its own, pressing left arrow key moves the focus back to the parent directory.

Note: I can go through the "root" items in the tree view using the down arrow without them expanding automatically. By root items I mean: my home directory, Trash, Network, volumes, File System etc.

I *suspect* (I'm not sure) this has something to do with the following commit and file:
    git log -p 85a79 -- thunar/thunar-tree-view.c

and searching for gtk_tree_view_expand_to_path there are lines such as these:

    -                {
    -                  /* just expand everything up to the row and its children */
    -                  gtk_tree_view_expand_to_path (GTK_TREE_VIEW (view), ancestor);
    -                }
    -              else if (!gtk_tree_view_row_expanded (GTK_TREE_VIEW (view), ancestor))
    -                {
    -                  /* just expand everything up to the row, but not the children */
    -                  parent = gtk_tree_path_copy (ancestor);
    -                  if (gtk_tree_path_up (parent))
    -                    gtk_tree_view_expand_to_path (GTK_TREE_VIEW (view), parent);
    -                  gtk_tree_path_free (parent);
    -                }
    
    +              gtk_tree_view_expand_to_path (GTK_TREE_VIEW (view), path);
    +              gtk_tree_view_set_cursor (GTK_TREE_VIEW (view), path, NULL, FALSE);
    +              gtk_tree_path_free (path);

So the gtk_tree_view_expand_to_path seemed to receive a different second argument at some point.

If this is the intended functionality, this bug might just as well be a feature request: a checkbox for the side pane tree options - "automatically expand focused directories". I'll create one if this is not a bug.