Can anyone help me figure out why hook_preprocess_links is failing to apply the correct CSS class to menu links?

Everything seems to be setup properly, except the javascript behavior is not getting attached to my menu links, because the CSS class is missing.
menu_minipanels.js looks for the class "'a.menu-minipanel-' + setting.mlid", but this is not being applied anywhere.

I'm assuming this is supposed to happen during menu_minipanels_preprocess_links(), but I'm not sure what could possibly be preventing it from happening.
Yes, I've cleared the cache.
The menu minipanel content is correctly rendered in the footer.

I've tried with Drupal core menu block, Nice Menus, and Menu Block module's menu block.
Does anyone know common causes for this issue?

I tried searching the queue already, alas, to no avail.