From your screenshot it looks like you imported your Evernote notes as HTML as the apostrophes are shown as HTML code (') in the note list preview.
HTML notes will use their own embedded HTML code for formatting. When you copy/paste the text of one of these notes into a new note the new note will be a Markdown note and the theme will apply. Duplicating the note creates another HTML note so the display issue still occurs.
However I would have thought that these HTML notes would have displayed just the same in previous versions of Joplin. You say "since the update...", what version of Joplin were you using before?