Thanks @jordesign. It went through the Site Health troubleshooting mode, and it is a conflict with my theme. But that conflict did not exist in 16.0.0, so something changed between then and now. I see 16.1.1 still has the conflict.
A few more things that I’ve discovered to try to help narrow down. The pre-population thing is post-specific. So if a comment is left on one post, then it only pre-populates the comment box on that same post.
