@cygnoir Are you sending yourself a preview of an old newsletter? If that's the case, it appears they stay formatted how they were when sent.
If not, can you go to Newsletter → Settings. Under Edit HTML, delete everything and hit save. Once you refresh that page, it should re-populate the HTML with the matching styles.
If you're still having issues, let me know. It's a hard feature to test from a development standpoint, so having user feedback is essential. Thanks!