It's a genuine Qt Bug. I reported it as https://bugreports.qt-project.org/browse/QTBUG-34981
Today we found the code responsible for it in
./qtwebkit/Source/WebKit/qt/WebCoreSupport/EditorClientQt.cpp
Around line 480 it says
#ifndef Q_WS_MAC
// We need to exclude checking for Alt because it is just a different Shift
if (!kevent->altKey())
#endif
shouldInsertText = true;
Apparently, Q_WS_MAC isn't defined on Mac Builds at this time - I think it's been deprecated in favor of Q_OS_MAC.S
Simply changing the statement to
shouldInsertText = true;
when compiling on Mac fixed the problem for us.