In the discussion regarding T195625: Implement a responsive layout for MonoBook, one user reported that they have issues with the tabs on new pages, either after following a red link or clicking the "Create page" tab. The problem is present in Firefox as well as Samsung Internet Browser and are independent of whether the user is logged in or not. However, for already existing pages, everything is fine.
They sent me the following screenshots:
I'm happy to relay any questions.