Print and PDF Export only generates "Letter / ANSI A" Format
MuseScore 3.0.1 is installed in an OpenSuse Tumbleweed OS (Qt Version 5.12.0)
1.) Select in Format > Page Settings > Page Size > A4 or A3 or any different to Letter format.
2.) Export PDF File or Print MuseScore Page on an A4 printer
3.) Obviously page settings are not applied to the output device. PDF Export has Letter Format (see Attachment). Result on a printer device also like on a Letter Format, that crops about 15 mm of the page header.
On a Windows 10 System it works. Maybe its a problem with the Linux System (Open Suse Tumbleweed) and its PDF engine and printer ps engine, as well.