Confirmed for me too – good get!
@hpardoe (or whoever else this might apply to), these commands should allow to you remove Qt5, install Qt4, then re-configure and build MRtrix3 – and hopefully solve your problem (until we find a better long-term solution):
pacman -Rcs mingw-w64_x86_64-qt5
pacman -Syu mingw-w64_x86_64-qt4
./configure
./build