Previously MediaWiki:Minerva.css was not loaded on mobile and was reserved for desktop Minerva (note in English Wikipedia for example there are rules for styling navboxes which are never present in mobile HTML).
In T237050 we accidentally added this module to the mobile site. This was unintentional and led to lots of confusion on wiki relating to hyphen styling.
Expected behaviour is MediaWiki:Minerva.css only loads for desktop Minerva users.