I can’t figure out why, but in my drop-down menu, only the newest child of a particular parent category is listed. I deactivated the multi-level nav plugin to see what would be listed, and without it, still only one child category is listed, so I don’t think it’s the plugin. This was also happening on 2.8.6
The example is the Beliefs category. There are six children, but only one shows up when you hover over Beliefs in the top nav bar.