Great, IMGUR’s down so I’m going to have to explain this with words, not pictures.
Assuming you haven’t changed any of the Layout Options for the Violin 2 part since you uploaded the file earlier, your Layout Options are set to use Flow Headers. When Flow Headers appear, you’ve got them set to Hide the Flow Title in Header. When it says Header it means the Header in the Master Page. So your Violin 2 part is using the First master page header, but making the Flow Title invisible. It’s then adding in a Flow Header below the regular Header. That Flow Header is the default Flow Header, which contains nothing but the Flow Title.
To change this, go to the Layout Options dialogue I screenshotted earlier, twice, select Violin 2 from the right panel, then set “Show Flow Headers” to “Never”. You’ll be rid of Flow Headers and back to the familiar header, which should perfectly match what you see in the First master page.