User blog comment:MisterWoodhouse/Discussions search is now available/@comment-28163602-20201221161244/@comment-821448-20210102053014

Sounds like a CSS issue. Something in the CSS codes, on one of the MediaWiki: pages, must have become broken with the UCP and is not supported any longer. Try the Fandom Discord server, they are usually really helpful and good at finding out what problems these are specifically! EDIT: It looks like a JavaScript (.JS) error specifically.