In T304188: Enable sticky header on Talk pages (article and user talk), we enabled the Vector 2022 sticky header on talk page namespaces where DiscussionTools visual enhancements were already enabled. Then in T325417: Make Usability Improvements avail. in all talk namespaces , we enabled DiscussionTools visual enhancements on all talk namespaces. We should update the sticky header behavior to match.
Documentation at https://www.mediawiki.org/wiki/Reading/Web/Desktop_Improvements/Features/Sticky_Header#Why_is_the_sticky_header_not_showing_on_a_certain_page? should be updated afterwards.