Thanks! I am probably not going to try to recreate the issue, because I don’t often use sections in posts. (In fact, this was the first time, so I thought there might be some glitch/issue/step unique to posts vs pages.)
I had just installed Redirection plugin by John Godley and Better Click to Tweet. I have those plug-ins on other sites and don’t remember this problem occurring on them, but like I’ve said, I don’t usually use sections in posts.
The only plug-ins I have on this site that aren’t on others are Automatic Responsive Tables by Andrew Rockwell and Category to Pages WUD. I did have a table in the section I was working on, but I’ve had tables in previous sections with that plug-in active without this issue arising.
Who knows. Even with changing my Heartbeat settings, WordPress is not automatically saving drafts as often as it should in general. So maybe it’s to do with that. It might have to do with something with my chron settings through my host?