Well, fudge, Geoff. When I turned off all plugins and switched to Twenty Sixteen I didn’t have the problem. After switching back and then turning off each plugin one at a time, I didn’t come up with a culprit.
Guess that means the problem is with the theme. I even made sure I had the latest version of our theme!
We only have a couple more of these unsaved organizer/venue events through the end of the year, so I suppose I’ll need to recreate the events as needed and then not post similar events moving forward until I have venue information confirmed.