Can't edit text of an embedded post

I created a post via JavaScript embedding and I edited the presentation of that embedded post.

The edit interface and post history both suggest that my edit was accepted, but when I (or anyone else) view the post, the original embed text is shown.

Is there a way to make the edit actually appear?

OK, it gets weirder. Some edits do show up; others don’t. Still figuring out the pattern.