Portions of menu tree not loading in some cases

I have a taxonomy vocabulary from which I'm creating a menu, showing 3 levels of 'categories' (taxonomy terms) with page nodes attached at the deepest levels. I'm using Taxonomy Menu and DHTML Menu. I have it configured to collapse all menu branches except the active trail. Like this:

category 1
- subcategory 1
- - subsubcategory 1
- - - node 1
- - - node 2 <-- you are here
- - - node 3
- - subsubcategory 2
- - subsubcategory 3
- subcategory 2
- subcategory 3
category 2
category 3

Using DHTML_Menu, I am avoiding loading any of the 'category' taxonomy pages, instead just expanding the menus. The only menu items that actually trigger a page load are the node page links. So, from the node 2 page ('you are here' in the above example), I can navigate to almost* any of the nodes in other branches of the menu tree (such as the children of category 2 or subcategory 2) with one page load. Just clicking the other categories to expand their children and drilling down to the desired node.

*But, if I try to expand subsubcategory 2 or 3, (any categories at the same level as the parent of the current node) they merely load the taxonomy page.

If I start on some page outside of the menu structure, and navigate in via the menu in its fully collapsed state, I can get to any of the nodes. But once on a node page within the menu, like node 2 above, I can't get to any of the nodes beneath the adjacent subsubcategories, without landing on the subsubcategories' taxonomy page first.

At first, I thought it had something to do with DHTML_Menu, and posted this issue. Arancaytar replied (#6 – 8) with some helpful explanations of what might be going on.