Figured it out. I had changed {{ .Content }} to {{ .Summary }} in the layouts/newsletter.html so that longer posts in the newsletter would have a summary and readers would have to click through to read the full thing. This was following this discussion with @jsonbecker and @toddgrotenhuis. It didn’t do anything in the newsletter, but I guess it changed the behavior of the <!-- more --> tag in a regular post.
I continue to not understand how any of this works. I simply undid the last thing I had changed in the template.