Bummer, deactivating other plugins and reverting to the 2015 theme didn’t have an effect. I have another event plugin installed called Event Organiser (that I am migrating away from and to The Events Calendar) that also has venue pages, but the permalink for that is events/venues so I don’t think there’s an issue there.
I’m wary to do anything crazy on a live site, so I will pull it down tonight and experiment further locally.