Never mind, figured it out. Adding:
<?php get_sidebar(); ?>
…at the proper place in the page.php for the theme sorted it. For whatever reason, the plugin I used worked on regular pages but not on forum pages. Editing the theme’s page.php directly made it work everywhere.
Additional info: I installed “What The File” on my staging area. Went to a regular page that now has a sidebar because of that other plugin. Compared the “What The File” references there to the “What The File” references for my main forum page. They’re identical – page.php and the same set of 5 “Template Parts”.
