Template:Ptag on mediawiki.org seems to create its own margin in the responsive MonoBook layout and prevents content from displaying on the right side of the page.
@Jc86035 Is this still an issue ? A screenshot might help people identify what exactly you were trying to deal with.
Guess this is either obsolete or I have no idea what this is about as no screenshot was added which allows comparison. In August 2020 things look like this: