Tertiary navigation problems

Great little module - just what I was looking for, aside from the wee problem I'm facing now. I'm hoping to use Menu Trim for tertiary navigation: I've got the site set so that both primary and secondary links are using Primary Links, and the Primary Links hierarchy is something like the following:

Primary Links
--A
----AA
------AAA
------AAB
----AB
--B
----BA
----BB

... and so on.

Other settings:

- In admin/settings/menu_trim, I've set the Primary Links menu to trim, and hide.
- The new (Menu trim) block for the Primary Links menu is visible in the sidebar.
- My theme displays primary and secondary links in the header.
- extending your example in a previous support issue, I've set every first and second level menu item to trim parents. That is, in the diagram above, the only items without the trim flag are AAA and AAB.

With these settings, I would expect the side block to appear only when AA is active in the header menu strip.

However, the block appears when I choose A (or B) - displaying A (or B) as the title of the block, and the secondary links as links. When I click on AA in the header menu, the sideblock changes to display what I want it to: AA as the title, and only AAA and AAB as links.

Whew. Sorry for the long explanation. Any hints to resolve it? Right now I'm working around it by configuring the (menu trim) block not to display on the A and B 'home' pages - but that's an extra step I'd rather not perform as I start building up the site content and navigation structure.

Cheers!