Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 horizontal dots at the bottom of the sidebar, scroll down to "Custom site panels" and add a new panel for https://www.reddit.com/ (or the URL for a specific subreddit).
@leocg Never mind, sorry for the trouble! I realized I had to move the bookmark from the tab itself, I used the bookmark page and moving a bookmark that is not in the main tab will force it to be renamed.
@leocg
Changing the Twitter theme does not solve the problem or make any visible change, even after refreshing the sidebar app. The page is still broken.
Additionally, I forgot to put this in the main post, but Spotify in the sidebar is also broken in the same way.