FontDatabase returns serif when asking for fixed font

Updated: I rebuilt Qt on the build system as completely dynamic and the same issue still occurs.

Using QFontDatabase I can output all the fonts on a system, building the application on one system and testing on another (both are running arch linux, build machine is an old outdated 32 bit install, test machine is 64 bit install). Using the sample code on the QFontDatabase page, the list of system fonts given is completely correct. All fonts on the system including monospace, oxygen mono etc. are listed. However, if I try to select a monospace font using
QFontDatabase::systemFont(QFontDatabase::FixedFont)
I am given Sans Serif (clearly not a monospace font) using the qt build (regardless if I make a static built application or dynamic built application) and Oxygen Mono using dynamic qt which built on my testing pc.

I've ran an strace of both and looked at what the difference between the font data files being read are between the two and I can't see anything different, both are loading all the font configuration and going through /usr/share/fonts and then the font files themselves are loaded, this is where the dynamic build loads oxygen mono and the static build loads sans serif.

Build machine is using fontconfig 2.11.1 and test machine is using fontconfig 2.12.0