This is a weird one, but I am not sure if I am missing something here. There used to be a location for the link to HTML files in content editor web parts. I cannot find the url now! I want to know which HTML file the web part is showing at the moment so I can modify it. It not under any of these: Appearance, Layout, Advanced.
The problem was SharePoint Online's content type editor had gone missing. So I managed to enable the feature back on again. As soon as I enabled the feature, the content editor edit panel went back to its previous status. I used this to do it: Content Editor Web Part missing on SharePoint Online