Hi Conor,
I'm glad to know you've found the origin of this issue. Believe me that I am including our own version of Qt with PixInsight on all platforms, including Linux, for strong reasons. I've seen lots of problems with Qt libraries provided by Linux distros, especially when PI is used under KDE. Our version of Qt is optimized for PI and compiled somewhat conservatively for the sake of stability; this is part of our QA procedures and we make sure PI runs out-of-the-box on Red Hat, Fedora and Ubuntu at least. I'll download Arch and will install it on a virtual machine to see where the problems are before releasing version 1.7.
In any case, a Linux user is always free to edit the PixInsight.sh launcher script to force PI Core use of distribution Qt libraries, although this is not recommended. If PI complains about unresolved symbols or lacking dependencies, please post the output of ldd here and we'll try to find a solution.