The CommonMark spec includes the ancient Setext headings like this:
Heading One
==========
While very few people use them these days, if you export and download all your Facebook data, all of the posts are in Setext format. If you open that folder with Lettera, nothing has any style shown. I saw that Lettera supports CommonMark, and since it’s already part of the CommonMark spec, it would be nice if Lettera also supported it.
Thanks for making Lettera available!
We had an early version of the Bear/Lettera editor that did support Setext headings. But it had many issues, and it confused the users as the syntax overlaps with the horizontal line.
So while we have CommonMark as our foundation, we have diverged in places where the standard makes the editor worse for people in general. We could have a stricter CommonMark mode for Lettera if there were common and strong use cases, but I don’t see that right now. But your case is at least noted.
I would use AI or some tool to convert all the files to use standard headings.