On the Configuring the Menus page it states: Left Sidebar Navigation: The menu that appears on the left-hand side of pages. In the UUA Theme, it’s based on the Main Navigation menu. However, updating the Main Navigation menu, such as removing an unwanted page, does NOT update the up the Left Sidebar menu. However, it does follow the page configuration on the Pages screen. Actually deleting the page, or setting Parent to "Main Page (no parent)" will remove that page from the Left Sidebar menu. In addition, the order displayed is the order on the Pages page, not the Main Navigation menu.
We fixed this in the documentation, but here are other comments that were lost when we updated UUA.org in August 2020.
I find it super-annoying that menu information is stored in two different places and updated in two different ways. But I guess that's just WordPress! I think we were assuming that we'd be keeping the main navigation menu and the page configuration menus settings in sync. I'll fix this (hope I get it right this time!).
May I suggest that you make the Theme match the documentation. Using the Pages screen to create a menu structure is the old way of doing it in WordPress. As more and more themes, like the UUA Theme, have multiple navigation menus, the Menus screen was introduced. For all of the other WordPress sites I maintain, I don't use the Pages screen for any organization. This allows for work-in-progress pages, or old pages to be left in place with no fear of them popping up is Pages driven menus. If the Left Sidebar Navigation can be driven from the Main Navigation Menu, or even better, its own menu, then the need to keep Pages in sync with anything else goes away.