Replying to myself:
The bugrepport said is is fixed for 5.10.1
https://bugreports.qt.io/browse/QTBUG-64121
And it can be made to work with correct folder permissions :
$ ls -ld /etc/config/
drwxr-xr-x 2 root root 4096 avril 1 11:15 /etc/config/
$ sudo chgrp pi /etc/config/
$ sudo chmod g+w /etc/config/
$ ls -ld /etc/cleandrop/
drwxrwxr-x 2 root pi 4096 avril 1 11:15 /etc/config/
resolving simlink doesn't helps
// resolve symlink
// https://bugreports.qt.io/browse/QTBUG-64121
QFileInfo info(_configuration_file);
if (info.isSymLink())
_configuration_file = info.symLinkTarget();
So I removed this fix, and fixed the folder permission and it worked.