I found while testing tst_qquicktextdocument::loadErrorPermissionDenied() in CI that if I copy a file to a temp directory and call

QFile::setPermissions(tmpPath, QFileDevice::Permissions{})

to make it unreadable, QFile can still read it afterwards. So maybe setPermissions() doesn't work, or QNX doesn't enforce permissions (this may not be a bug then), or QFile::open() can bypass them somehow.