when uniting painter paths, sometimes the resulting path is "open" and parts of it get erased.
this code should show the issue (at the end the same ellipses are drawn using add for comparison):
QPainterPath p, path;

path=QPainterPath();
p=QPainterPath();
p.addEllipse( 284 , 268 , 30, 30);
path|=p;
p=QPainterPath();
p.addEllipse( 303 , 267 , 30, 30);
path|=p;
p=QPainterPath();
p.addEllipse( 305 , 267 , 30, 30);
path|=p;
p=QPainterPath();
p.addEllipse( 303 , 267 , 30, 30);
path|=p;
p=QPainterPath();
p.addEllipse( 292 , 267 , 30, 30);
path|=p;
painter.drawPath(path);

p=QPainterPath();
p.addEllipse( 284 , 238 , 30, 30);
p.addEllipse( 303 , 237 , 30, 30);
p.addEllipse( 305 , 237 , 30, 30);
p.addEllipse( 303 , 237 , 30, 30);
p.addEllipse( 292 , 237 , 30, 30);
painter.drawPath(p);

i've made two attachments. the first gives the result i get and the second is demonstration code. just click and move the mouse slightly back and forth. it doesn't happen, if moving fast.