I believe that it is the import. An ENEX file can be imported as either HTML or Markdown.
If imported as HTML a lot of the formatting used in Evernote will be retained but in order to achieve that the note is created using, as the import suggests, HTML. It will make the notes very difficult to edit, as you have found.
If imported as Markdown a lot of the formatting (such as the highlights in your screenshot) will be lost. That is because Markdown is limited in the formatting it can apply. The advantage is that the note will be easy to edit. That is why Joplin was created as a Markdown-based note taking app. It was so that a note was not in a proprietary format, was easy to read without the app, and the information it contained could easily be moved elsewhere.