We are modifying the HTML markup for the sticky header. Per our frontend stable policy this is not a stable interface but as a courtesy I wanted to let maintainers of user styles know they will need to update their CSS.
On short term, please update your selector to #vector-sticky-header .mw-page-title-main (please note that while #vector-sticky-header is also not considered a stable interface it is less likely to change).
https://global-search.toolforge.org/?q=%5C.vector-sticky-header-context-bar-primary®ex=1&namespaces=&title=
As a reminder per our stable policy, please request new APIs if this functionality is important to avoid it breaking again (see
https://www.mediawiki.org/wiki/Stable_interface_policy/Frontend#Communication for more information)