Just in case anyone else has run up against this, to solve this issue I had to change the CSS property “display” of “#wprmenu_menu ul” from “block” to “inline-block”.
I’m not sure if this was an issue introduced when we updated to WordPress 4.5 or not, but that’s the only thing that had changed around the time that the issue had arose. Everything had been working fine previous to that, and the plugin hadn’t been updated since the issue arose.