And it looks like the KDE people already have a fix for the bug. They create a new plugin that knows how to deal with their KIconEngine and according to the tests, it fixes the problems!
Wow fast work on their part! Here is the link with the changes needed.
https://phabricator.kde.org/D16305
So it looks like it was not a Qt bug at all but a serialization/deserialization issue with Icon data probably in an Icon cache of some sort.
Not sure when this will fix will appear in a KDE release, but it is nice to know a fix now exists.