On Kubuntu 12.04 LTS.
For some reason, Qt applications (Kate, Konsole, etc.) were loading the ISO8859-1 Compose key file, instead of UTF-8. Other applications (Chrome and Java apps, for example) were working correctly.
You can tell if this is happening by typing the following key combination:
<Compose_key> then <underscore> then <a>
If you get ā (an a with a macron), you're using the UTF-8 file. If you get ã (an a with a tilde), you're using the ISO8859-1 file.
The only way I figured out to fix the problem was to install qt4-qtconfig and change the "Default Input Method" to xim instead of ibus. It must be that ibus is looking in the wrong spot for the Console file.
Hope this helps someone else (and I hope the Qt developers fix the problem for Qt5).