And here's a comment from the same Qt developer on how to get a bug fixed if you have a commercial license:

Originally Posted By: qt team member
If you have a commercial license with support, then use that channel to get help, possibly workarounds and maybe higher priority for the bugs. No guarantees that bugs will get fixed, but at least you’ll have somewhere to go


This seems to tally with the comments in the bug tracker for (what turned out to be) a long standing bug in Qt.

Adrian