Tested on powerpc64, applications that use qt5 do not seem to recognize the mouse wheel. Enable debug logging, I dos see it getting 'legacy' button 4/button 5 events, but they don't seem to propagate up to the rest of the framework as a wheel event.
With power9 this is more relevant (my initial reaction was: meh, old iMacs). I know KDE Plasma 5 Just Works(tm) with Debian on the Power9 Talos machines, since I installed it there at ELCE. For this, I think we're going to need to borrow a box (or at least access to one) to sort it out. Something to talk about at FOSDEM, I warrant (with the OpenPower people in the room).
This turns out to be more than a FreeBSD issue. The Adelie folks also see this problem on Linux. So it's a big-endian issue in general (spans 32-bit and 64-bit BE).
Seems it's an XInput-related issue. Setting "QT_XCB_NO_XI2=y" in my environment lets me use the mouse wheel.
I'm going to close this one as "upstream issue with known workaround". I don't think there's much we can do (except advertise that environment variable).